Make the keyboard reappear when you tap on a text field after having dismissed the keyboard.

- the keyboard service now has a showByRequest() method that invokes Android's InputMethodManager's imm.showSoftInput with no flags set.
- the Input component calls through to the keyboard service's showByRequest() when it receives a pointer down.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1089163005

framework/components/input.dart

@@ -8,6 +8,7 @@ import '../editing/keyboard.dart';
 import '../fn.dart';
 import '../theme/colors.dart';
 import '../theme/typography.dart' as typography;
+import 'dart:sky' as sky;
 
 typedef void ValueChanged(value);
 
@@ -82,10 +83,13 @@ class Input extends Component {
 
     children.add(new EditableText(value: _editableValue, focused: focused));
 
-    return new Container(
-      style: _style,
-      inlineStyle: focused ? _focusedInlineStyle : null,
-      children: children
+    return new EventListenerNode(
+      new Container(
+        style: _style,
+        inlineStyle: focused ? _focusedInlineStyle : null,
+        children: children
+      ),
+      onPointerDown: (sky.Event e) => keyboard.showByRequest()
     );
   }
 }