Renamed Positioned constructor. (#5927)

Renames stretch constructor to fill for better consistency with
the rest of the framework.

dev/manual_tests/card_collection.dart

@@ -363,7 +363,7 @@ class CardCollectionState extends State<CardCollection> {
     // size of the background,card Stack will be based only on the card. The
     // Viewport ensures that when the card's resize animation occurs, the
     // background (text and icons) will just be clipped, not resized.
-    Widget background = new Positioned.stretch(
+    Widget background = new Positioned.fill(
       child: new Container(
         margin: const EdgeInsets.all(4.0),
         child: new Viewport(

examples/flutter_gallery/lib/demo/cards_demo.dart

@@ -61,7 +61,7 @@ class TravelDestinationItem extends StatelessWidget {
               height: 184.0,
               child: new Stack(
                 children: <Widget>[
-                  new Positioned.stretch(
+                  new Positioned.fill(
                     child: new Image.asset(
                       destination.assetName,
                       fit: ImageFit.cover

packages/flutter/lib/src/material/grid_tile.dart

@@ -38,7 +38,7 @@ class GridTile extends StatelessWidget {
       return child;
 
     final List<Widget> children = <Widget>[
-      new Positioned.stretch(
+      new Positioned.fill(
         child: child
       )
     ];

packages/flutter/lib/src/material/tooltip.dart

@@ -265,7 +265,7 @@ class _TooltipOverlay extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    return new Positioned.stretch(
+    return new Positioned.fill(
       child: new IgnorePointer(
         child: new CustomSingleChildLayout(
           delegate: new _TooltipPositionDelegate(

packages/flutter/lib/src/widgets/basic.dart

@@ -1519,7 +1519,7 @@ class Positioned extends ParentDataWidget<Stack> {
 
   /// Creates a Positioned object with [left], [top], [right], and [bottom] set
   /// to 0.0 unless a value for them is passed.
-  Positioned.stretch({
+  Positioned.fill({
     Key key,
     Widget child,
     this.left: 0.0,