Make UINode's key a String

We need the key to be a String even though we claimed we could support any
Object. Also, clean up some style nits including shortening the |root| getters
on OneChildRenderNodeWrappers.

Fixes https://github.com/domokit/sky_sdk/issues/26.

TBR=ianh@google.com

Review URL: https://codereview.chromium.org/1173293005.

examples/stocks2/lib/stock_arrow.dart

@@ -12,7 +12,7 @@ import 'package:sky/widgets/basic.dart';
 
 class StockArrow extends Component {
 
-  StockArrow({ Object key, this.percentChange }) : super(key: key);
+  StockArrow({ String key, this.percentChange }) : super(key: key);
 
   final double percentChange;
 

examples/stocks2/lib/stock_list.dart

@@ -11,7 +11,7 @@ import 'stock_row.dart';
 class Stocklist extends FixedHeightScrollable {
 
   Stocklist({
-    Object key,
+    String key,
     this.stocks,
     this.query
   }) : super(itemHeight: StockRow.kHeight, key: key);

examples/stocks2/lib/stock_menu.dart

@@ -10,7 +10,7 @@ import 'package:sky/framework/theme/view_configuration.dart';
 class StockMenu extends Component {
 
   StockMenu({
-    Object key,
+    String key,
     this.controller,
     this.autorefresh: false,
     this.onAutorefreshChanged

examples/widgets/spinning_mixed.dart

@@ -24,7 +24,7 @@ void addFlexChildSolidColor(RenderFlex parent, sky.Color backgroundColor, { int
 
 // Solid colour, Widget version
 class Rectangle extends Component {
-  Rectangle(this.color, { Object key }) : super(key: key);
+  Rectangle(this.color, { String key }) : super(key: key);
   final Color color;
   UINode build() {
     return new Flexible(

sdk/lib/editing2/editable_text.dart

@@ -9,7 +9,7 @@ import 'editable_string.dart';
 
 class EditableText extends Component {
 
-  EditableText({Object key, this.value, this.focused})
+  EditableText({String key, this.value, this.focused})
       : super(key: key, stateful: true);
 
   // static final Style _cursorStyle = new Style('''

sdk/lib/editing2/input.dart

@@ -13,7 +13,7 @@ typedef void ValueChanged(value);
 
 class Input extends Component {
 
-  Input({Object key,
+  Input({String key,
          this.placeholder,
          this.onChanged,
          this.focused})

sdk/lib/widgets/animated_component.dart

@@ -18,7 +18,7 @@ class _AnimationEntry {
 
 abstract class AnimatedComponent extends Component {
 
-  AnimatedComponent({ Object key }) : super(key: key, stateful: true);
+  AnimatedComponent({ String key }) : super(key: key, stateful: true);
 
   void syncFields(AnimatedComponent source) { }
 

sdk/lib/widgets/basic.dart

@@ -1,3 +1,6 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
 
 import 'package:vector_math/vector_math.dart';
 
@@ -18,10 +21,10 @@ export 'ui_node.dart' show UINode, Component, App, EventListenerNode, ParentData
 // PAINTING NODES
 
 class Opacity extends OneChildRenderObjectWrapper {
-  Opacity({ this.opacity, UINode child, Object key })
-    : super(child: child, key: key);
+  Opacity({ String key, this.opacity, UINode child })
+    : super(key: key, child: child);
 
-  RenderOpacity get root { RenderOpacity result = super.root; return result; }
+  RenderOpacity get root => super.root;
   final double opacity;
 
   RenderOpacity createNode() => new RenderOpacity(opacity: opacity);
@@ -34,10 +37,10 @@ class Opacity extends OneChildRenderObjectWrapper {
 
 class DecoratedBox extends OneChildRenderObjectWrapper {
 
-  DecoratedBox({ this.decoration, UINode child, Object key })
-    : super(child: child, key: key);
+  DecoratedBox({ String key, this.decoration, UINode child })
+    : super(key: key, child: child);
 
-  RenderDecoratedBox get root { RenderDecoratedBox result = super.root; return result; }
+  RenderDecoratedBox get root => super.root;
   final BoxDecoration decoration;
 
   RenderDecoratedBox createNode() => new RenderDecoratedBox(decoration: decoration);
@@ -51,10 +54,10 @@ class DecoratedBox extends OneChildRenderObjectWrapper {
 
 class CustomPaint extends OneChildRenderObjectWrapper {
 
-  CustomPaint({ this.callback, this.token, UINode child, Object key })
-    : super(child: child, key: key);
+  CustomPaint({ String key, this.callback, this.token, UINode child })
+    : super(key: key, child: child);
 
-  RenderCustomPaint get root { RenderCustomPaint result = super.root; return result; }
+  RenderCustomPaint get root => super.root;
   final CustomPaintCallback callback;
   final dynamic token;  // set this to be repainted automatically when the token changes
 
@@ -75,20 +78,18 @@ class CustomPaint extends OneChildRenderObjectWrapper {
 }
 
 class ClipRect extends OneChildRenderObjectWrapper {
+  ClipRect({ String key, UINode child })
+    : super(key: key, child: child);
 
-  ClipRect({ UINode child, Object key })
-    : super(child: child, key: key);
-
-  RenderClipRect get root { RenderClipRect result = super.root; return result; }
+  RenderClipRect get root => super.root;
   RenderClipRect createNode() => new RenderClipRect();
 }
 
 class ClipOval extends OneChildRenderObjectWrapper {
+  ClipOval({ String key, UINode child })
+    : super(key: key, child: child);
 
-  ClipOval({ UINode child, Object key })
-    : super(child: child, key: key);
-
-  RenderClipOval get root { RenderClipOval result = super.root; return result; }
+  RenderClipOval get root => super.root;
   RenderClipOval createNode() => new RenderClipOval();
 }
 
@@ -97,10 +98,10 @@ class ClipOval extends OneChildRenderObjectWrapper {
 
 class Transform extends OneChildRenderObjectWrapper {
 
-  Transform({ this.transform, UINode child, Object key })
-    : super(child: child, key: key);
+  Transform({ String key, this.transform, UINode child })
+    : super(key: key, child: child);
 
-  RenderTransform get root { RenderTransform result = super.root; return result; }
+  RenderTransform get root => super.root;
   final Matrix4 transform;
 
   RenderTransform createNode() => new RenderTransform(transform: transform);
@@ -114,10 +115,10 @@ class Transform extends OneChildRenderObjectWrapper {
 
 class Padding extends OneChildRenderObjectWrapper {
 
-  Padding({ this.padding, UINode child, Object key })
-    : super(child: child, key: key);
+  Padding({ String key, this.padding, UINode child })
+    : super(key: key, child: child);
 
-  RenderPadding get root { RenderPadding result = super.root; return result; }
+  RenderPadding get root => super.root;
   final EdgeDims padding;
 
   RenderPadding createNode() => new RenderPadding(padding: padding);
@@ -130,26 +131,23 @@ class Padding extends OneChildRenderObjectWrapper {
 }
 
 class Center extends OneChildRenderObjectWrapper {
+  Center({ String key, UINode child })
+    : super(key: key, child: child);
 
-  Center({ UINode child, Object key })
-    : super(child: child, key: key);
-
-  RenderPositionedBox get root { RenderPositionedBox result = super.root; return result; }
-
+  RenderPositionedBox get root => super.root;
   RenderPositionedBox createNode() => new RenderPositionedBox();
-
 }
 
 class SizedBox extends OneChildRenderObjectWrapper {
 
   SizedBox({
+    String key,
     this.width,
     this.height,
-    UINode child,
-    Object key
-  }) : super(child: child, key: key);
+    UINode child
+  }) : super(key: key, child: child);
 
-  RenderConstrainedBox get root { RenderConstrainedBox result = super.root; return result; }
+  RenderConstrainedBox get root => super.root;
 
   final double width;
   final double height;
@@ -174,10 +172,11 @@ class SizedBox extends OneChildRenderObjectWrapper {
 
 class ConstrainedBox extends OneChildRenderObjectWrapper {
 
-  ConstrainedBox({ this.constraints, UINode child, Object key })
-    : super(child: child, key: key);
+  ConstrainedBox({ String key, this.constraints, UINode child })
+    : super(key: key, child: child);
+
+  RenderConstrainedBox get root => super.root;
 
-  RenderConstrainedBox get root { RenderConstrainedBox result = super.root; return result; }
   final BoxConstraints constraints;
 
   RenderConstrainedBox createNode() => new RenderConstrainedBox(additionalConstraints: constraints);
@@ -190,21 +189,19 @@ class ConstrainedBox extends OneChildRenderObjectWrapper {
 }
 
 class ShrinkWrapWidth extends OneChildRenderObjectWrapper {
+  ShrinkWrapWidth({ String key, UINode child })
+    : super(key: key, child: child);
 
-  ShrinkWrapWidth({ UINode child, Object key }) : super(child: child, key: key);
-
-  RenderShrinkWrapWidth get root { RenderShrinkWrapWidth result = super.root; return result; }
-
+  RenderShrinkWrapWidth get root => super.root;
   RenderShrinkWrapWidth createNode() => new RenderShrinkWrapWidth();
-
 }
 
 class SizeObserver extends OneChildRenderObjectWrapper {
 
-  SizeObserver({ this.callback, UINode child, Object key })
-    : super(child: child, key: key);
+  SizeObserver({ String key, this.callback, UINode child })
+    : super(key: key, child: child);
 
-  RenderSizeObserver get root { RenderSizeObserver result = super.root; return result; }
+  RenderSizeObserver get root => super.root;
   final SizeChangedCallback callback;
 
   RenderSizeObserver createNode() => new RenderSizeObserver(callback: callback);
@@ -227,7 +224,7 @@ class SizeObserver extends OneChildRenderObjectWrapper {
 class Container extends Component {
 
   Container({
-    Object key,
+    String key,
     this.child,
     this.constraints,
     this.decoration,
@@ -287,48 +284,47 @@ class Container extends Component {
 // LAYOUT NODES
 
 class Block extends MultiChildRenderObjectWrapper {
-
-  Block(List<UINode> children, { Object key })
+  Block(List<UINode> children, { String key })
     : super(key: key, children: children);
 
-  RenderBlock get root { RenderBlock result = super.root; return result; }
+  RenderBlock get root => super.root;
   RenderBlock createNode() => new RenderBlock();
-
 }
 
 class Stack extends MultiChildRenderObjectWrapper {
-
-  Stack(List<UINode> children, { Object key })
+  Stack(List<UINode> children, { String key })
     : super(key: key, children: children);
 
-  RenderStack get root { RenderStack result = super.root; return result; }
+  RenderStack get root => super.root;
   RenderStack createNode() => new RenderStack();
-
 }
 
 class Positioned extends ParentDataNode {
   Positioned({
+    String key,
     UINode child,
     double top,
     double right,
     double bottom,
     double left
-  }) : super(content, new StackParentData()..top = top
-                                           ..right = right
-                                           ..bottom = bottom
-                                           ..left = left);
+  }) : super(content,
+             new StackParentData()..top = top
+                                  ..right = right
+                                  ..bottom = bottom
+                                  ..left = left,
+             key: key);
 }
 
 class Flex extends MultiChildRenderObjectWrapper {
 
   Flex(List<UINode> children, {
-    Object key,
+    String key,
     this.direction: FlexDirection.horizontal,
     this.justifyContent: FlexJustifyContent.flexStart,
     this.alignItems: FlexAlignItems.center
   }) : super(key: key, children: children);
 
-  RenderFlex get root { RenderFlex result = super.root; return result; }
+  RenderFlex get root => super.root;
   RenderFlex createNode() => new RenderFlex(direction: this.direction);
 
   final FlexDirection direction;
@@ -345,15 +341,15 @@ class Flex extends MultiChildRenderObjectWrapper {
 }
 
 class Flexible extends ParentDataNode {
-  Flexible({ UINode child, int flex: 1, Object key })
+  Flexible({ String key, UINode child, int flex: 1 })
     : super(child, new FlexBoxParentData()..flex = flex, key: key);
 }
 
 class Paragraph extends RenderObjectWrapper {
 
-  Paragraph({ Object key, this.text, this.style }) : super(key: key);
+  Paragraph({ String key, this.text, this.style }) : super(key: key);
 
-  RenderParagraph get root { RenderParagraph result = super.root; return result; }
+  RenderParagraph get root => super.root;
   RenderParagraph createNode() => new RenderParagraph(text: text, style: style);
 
   final String text;
@@ -383,12 +379,12 @@ class Text extends Component {
 class Image extends RenderObjectWrapper {
 
   Image({
-    Object key,
+    String key,
     this.src,
     this.size
   }) : super(key: key);
 
-  RenderImage get root { RenderImage result = super.root; return result; }
+  RenderImage get root => super.root;
   RenderImage createNode() => new RenderImage(this.src, this.size);
 
   final String src;

sdk/lib/widgets/button_base.dart

@@ -6,7 +6,7 @@ import 'basic.dart';
 
 abstract class ButtonBase extends Component {
 
-  ButtonBase({ Object key, this.highlight: false }) : super(key: key);
+  ButtonBase({ String key, this.highlight: false }) : super(key: key);
 
   bool highlight;
 

sdk/lib/widgets/checkbox.dart

@@ -20,7 +20,7 @@ const double _kEdgeRadius = 1.0;
 class Checkbox extends Toggleable {
 
   Checkbox({
-    Object key,
+    String key,
     bool value,
     ValueChanged onChanged
   }) : super(key: key, value: value, onChanged: onChanged);

sdk/lib/widgets/drawer.dart

@@ -109,7 +109,7 @@ class DrawerController {
 class Drawer extends AnimatedComponent {
 
   Drawer({
-    Object key,
+    String key,
     this.controller,
     this.children,
     this.level: 0

sdk/lib/widgets/drawer_header.dart

@@ -8,7 +8,7 @@ import 'basic.dart';
 
 class DrawerHeader extends Component {
 
-  DrawerHeader({ Object key, this.children }) : super(key: key);
+  DrawerHeader({ String key, this.children }) : super(key: key);
 
   final List<UINode> children;
 

sdk/lib/widgets/fixed_height_scrollable.dart

@@ -12,7 +12,7 @@ import 'scrollable.dart';
 
 abstract class FixedHeightScrollable extends Scrollable {
 
-  FixedHeightScrollable({ this.itemHeight, Color backgroundColor, Object key })
+  FixedHeightScrollable({ String key, this.itemHeight, Color backgroundColor })
       : super(key: key, backgroundColor: backgroundColor) {
     assert(itemHeight != null);
   }

sdk/lib/widgets/floating_action_button.dart

@@ -15,8 +15,7 @@ const double _kSize = 56.0;
 
 class FloatingActionButton extends ButtonBase {
 
-  FloatingActionButton({ Object key, this.child })
-      : super(key: key);
+  FloatingActionButton({ String key, this.child }) : super(key: key);
 
   final UINode child;
 

sdk/lib/widgets/ink_well.dart

@@ -100,10 +100,9 @@ class RenderInkWell extends RenderProxyBox {
 }
 
 class InkWell extends OneChildRenderObjectWrapper {
-  InkWell({ UINode child, Object key })
-    : super(child: child, key: key);
-
-  RenderInkWell get root { RenderInkWell result = super.root; return result; }
+  InkWell({ String key, UINode child })
+    : super(key: key, child: child);
 
+  RenderInkWell get root => super.root;
   RenderInkWell createNode() => new RenderInkWell();
 }

sdk/lib/widgets/material.dart

@@ -11,7 +11,7 @@ import 'basic.dart';
 class Material extends Component {
 
   Material({
-    Object key,
+    String key,
     this.child,
     this.edge: MaterialEdge.card,
     this.level: 0,

sdk/lib/widgets/menu_divider.dart

@@ -5,7 +5,7 @@
 import 'basic.dart';
 
 class MenuDivider extends Component {
-  MenuDivider({ Object key }) : super(key: key);
+  MenuDivider({ String key }) : super(key: key);
 
   UINode build() {
     return new Container(

sdk/lib/widgets/menu_item.dart

@@ -23,7 +23,8 @@ const BoxDecoration _kHighlightBoring = const BoxDecoration(
 );
 
 class MenuItem extends ButtonBase {
-  MenuItem({ Object key, this.icon, this.children, this.onGestureTap }) : super(key: key);
+  MenuItem({ String key, this.icon, this.children, this.onGestureTap })
+    : super(key: key);
 
   String icon;
   List<UINode> children;

sdk/lib/widgets/modal_overlay.dart

@@ -7,7 +7,7 @@ import 'ui_node.dart';
 
 class ModalOverlay extends Component {
 
-  ModalOverlay({ Object key, this.children, this.onDismiss }) : super(key: key);
+  ModalOverlay({ String key, this.children, this.onDismiss }) : super(key: key);
 
   // static final Style _style = new Style('''
   //   position: absolute;

sdk/lib/widgets/popup_menu.dart

@@ -54,7 +54,7 @@ class PopupMenuController {
 
 class PopupMenu extends AnimatedComponent {
 
-  PopupMenu({ Object key, this.controller, this.items, this.level })
+  PopupMenu({ String key, this.controller, this.items, this.level })
       : super(key: key) {
     _painter = new BoxPainter(new BoxDecoration(
       backgroundColor: Grey[50],
@@ -94,7 +94,10 @@ class PopupMenu extends AnimatedComponent {
     int i = 0;
     List<UINode> children = new List.from(items.map((List<UINode> item) {
       double opacity = _opacityFor(i);
-      return new PopupMenuItem(key: i++, children: item, opacity: opacity);
+      // TODO(abarth): Using |i| for the key here seems wrong.
+      return new PopupMenuItem(key: (i++).toString(),
+                               children: item
+                               opacity: opacity);
     }));
 
     return new Opacity(

sdk/lib/widgets/popup_menu_item.dart

@@ -6,7 +6,7 @@ import 'basic.dart';
 import 'ink_well.dart';
 
 class PopupMenuItem extends Component {
-  PopupMenuItem({ Object key, this.children, this.opacity}) : super(key: key);
+  PopupMenuItem({ String key, this.children, this.opacity}) : super(key: key);
 
   final List<UINode> children;
   final double opacity;

sdk/lib/widgets/radio.dart

@@ -14,7 +14,7 @@ typedef void ValueChanged(value);
 class Radio extends ButtonBase {
 
   Radio({
-    Object key,
+    String key,
     this.value,
     this.groupValue,
     this.onChanged

sdk/lib/widgets/raised_button.dart

@@ -14,7 +14,7 @@ enum RaisedButtonTheme { light, dark }
 class RaisedButton extends ButtonBase {
 
   RaisedButton({
-    Object key,
+    String key,
     this.child,
     this.enabled: true,
     this.onPressed,

sdk/lib/widgets/scaffold.dart

@@ -159,7 +159,7 @@ class Scaffold extends RenderObjectWrapper {
   //   ${typography.black.body1};''');
 
   Scaffold({
-    Object key,
+    String key,
     UINode toolbar,
     UINode body,
     UINode statusBar,
@@ -178,7 +178,7 @@ class Scaffold extends RenderObjectWrapper {
   UINode _drawer;
   UINode _floatingActionButton;
 
-  RenderScaffold get root { RenderScaffold result = super.root; return result; }
+  RenderScaffold get root => super.root;
   RenderScaffold createNode() => new RenderScaffold();
 
   void insert(RenderObjectWrapper child, ScaffoldSlots slot) {

sdk/lib/widgets/scrollable.dart

@@ -26,7 +26,8 @@ abstract class ScrollClient {
 
 abstract class Scrollable extends Component {
 
-  Scrollable({ Object key, Color this.backgroundColor }) : super(key: key, stateful: true);
+  Scrollable({ String key, Color this.backgroundColor })
+    : super(key: key, stateful: true);
 
   Color backgroundColor;
 

sdk/lib/widgets/switch.dart

@@ -29,7 +29,7 @@ class Switch extends Toggleable {
   // TODO(jackson): Hit-test the switch so that it can respond to both taps and swipe gestures
 
   Switch({
-    Object key,
+    String key,
     bool value,
     ValueChanged onChanged
   }) : super(key: key, value: value, onChanged: onChanged);

sdk/lib/widgets/toggleable.dart

@@ -16,7 +16,7 @@ const double _kCheckDuration = 200.0;
 abstract class Toggleable extends AnimatedComponent {
 
   Toggleable({
-    Object key,
+    String key,
     this.value,
     this.onChanged
   }) : super(key: key) {

sdk/lib/widgets/ui_node.dart

@@ -15,18 +15,13 @@ export '../rendering/box.dart' show BoxConstraints, BoxDecoration, Border, Borde
 export '../rendering/flex.dart' show FlexDirection;
 export '../rendering/object.dart' show Point, Size, Rect, Color, Paint, Path;
 
-
-// final sky.Tracing _tracing = sky.window.tracing;
-
 final bool _shouldLogRenderDuration = false;
 
-/*
- * All Effen nodes derive from UINode. All nodes have a _parent, a _key and
- * can be sync'd.
- */
+// All Effen nodes derive from UINode. All nodes have a _parent, a _key and
+// can be sync'd.
 abstract class UINode {
 
-  UINode({ Object key }) {
+  UINode({ String key }) {
     _key = key == null ? "$runtimeType" : "$runtimeType-$key";
     assert(this is AbstractUINodeRoot || _inRenderDirtyComponents); // you should not build the UI tree ahead of time, build it only during build()
   }
@@ -157,7 +152,8 @@ abstract class UINode {
 // stylistic information, etc.
 abstract class TagNode extends UINode {
 
-  TagNode(UINode content, { Object key }) : this.content = content, super(key: key);
+  TagNode(UINode content, { String key })
+    : this.content = content, super(key: key);
 
   UINode content;
 
@@ -178,7 +174,8 @@ abstract class TagNode extends UINode {
 }
 
 class ParentDataNode extends TagNode {
-  ParentDataNode(UINode content, this.parentData, { Object key }): super(content, key: key);
+  ParentDataNode(UINode content, this.parentData, { String key })
+    : super(content, key: key);
   final ParentData parentData;
 }
 
@@ -275,14 +272,11 @@ class EventListenerNode extends TagNode  {
 
 abstract class Component extends UINode {
 
-  Component({ Object key, bool stateful })
+  Component({ String key, bool stateful })
       : _stateful = stateful != null ? stateful : false,
         _order = _currentOrder + 1,
         super(key: key);
 
-  Component.fromArgs(Object key, bool stateful)
-      : this(key: key, stateful: stateful);
-
   static Component _currentlyBuilding;
   bool get _isBuilding => _currentlyBuilding == this;
 
@@ -341,14 +335,13 @@ abstract class Component extends UINode {
   final int _order;
   static int _currentOrder = 0;
 
-  /* There are three cases here:
-   * 1) Building for the first time:
-   *      assert(_built == null && old == null)
-   * 2) Re-building (because a dirty flag got set):
-   *      assert(_built != null && old == null)
-   * 3) Syncing against an old version
-   *      assert(_built == null && old != null)
-   */
+  // There are three cases here:
+  // 1) Building for the first time:
+  //      assert(_built == null && old == null)
+  // 2) Re-building (because a dirty flag got set):
+  //      assert(_built != null && old == null)
+  // 3) Syncing against an old version
+  //      assert(_built == null && old != null)
   void _sync(UINode old, dynamic slot) {
     assert(_built == null || old == null);
     assert(!_disqualifiedFromEverAppearingAgain);
@@ -456,18 +449,14 @@ void _scheduleComponentForRender(Component c) {
 }
 
 
-/*
- * RenderObjectWrappers correspond to a desired state of a RenderObject.
- * They are fully immutable, with one exception: A UINode which is a
- * Component which lives within an MultiChildRenderObjectWrapper's
- * children list, may be replaced with the "old" instance if it has
- * become stateful.
- */
+// RenderObjectWrappers correspond to a desired state of a RenderObject.
+// They are fully immutable, with one exception: A UINode which is a
+// Component which lives within an MultiChildRenderObjectWrapper's
+// children list, may be replaced with the "old" instance if it has
+// become stateful.
 abstract class RenderObjectWrapper extends UINode {
 
-  RenderObjectWrapper({
-    Object key
-  }) : super(key: key);
+  RenderObjectWrapper({ String key }) : super(key: key);
 
   RenderObject createNode();
 
@@ -524,7 +513,8 @@ abstract class RenderObjectWrapper extends UINode {
 
 abstract class OneChildRenderObjectWrapper extends RenderObjectWrapper {
 
-  OneChildRenderObjectWrapper({ UINode child, Object key }) : _child = child, super(key: key);
+  OneChildRenderObjectWrapper({ UINode child, String key })
+    : _child = child, super(key: key);
 
   UINode _child;
   UINode get child => _child;
@@ -564,11 +554,9 @@ abstract class MultiChildRenderObjectWrapper extends RenderObjectWrapper {
   // In MultiChildRenderObjectWrapper subclasses, slots are RenderObject nodes
   // to use as the "insert before" sibling in ContainerRenderObjectMixin.add() calls
 
-  MultiChildRenderObjectWrapper({
-    Object key,
-    List<UINode> children
-  }) : this.children = children == null ? const [] : children,
-       super(key: key) {
+  MultiChildRenderObjectWrapper({ String key, List<UINode> children })
+    : this.children = children == null ? const [] : children,
+      super(key: key) {
     assert(!_debugHasDuplicateIds());
   }