Rationalise the Key API.

Add a way of having keys based on numeric types or DateTimes by having a ValueKey<T> class.
Remove the redundant ways of declaring things, except for leaving one shorthand -- you can say `new Key(s)` instead of `new ValueKey<String>(s)`.

examples/fitness/lib/fitness_item.dart

@@ -30,7 +30,7 @@ abstract class FitnessItemRow extends Component {
 
   FitnessItemRow({ FitnessItem item, this.onDismissed })
    : this.item = item,
-     super(key: new Key(item.when.toString())) {
+     super(key: new ValueKey<DateTime>(item.when)) {
     assert(onDismissed != null);
   }
 

examples/mine_digger/lib/main.dart

@@ -142,7 +142,7 @@ class MineDiggerApp extends App {
         new Row(
           row,
           justifyContent: FlexJustifyContent.center,
-          key: new Key.stringify(iy)
+          key: new ValueKey<int>(iy)
         )
       );
     }

examples/widgets/card_collection.dart

@@ -13,7 +13,7 @@ class CardModel {
   double height;
   Color color;
   String get label => "Item $value";
-  Key get key => new Key.fromObjectIdentity(this);
+  Key get key => new ObjectKey(this);
 }
 
 class CardCollectionApp extends App {

examples/widgets/ensure_visible.dart

@@ -12,7 +12,7 @@ class CardModel {
   double height;
   Color color;
   String get label => "Card $value";
-  Key get key => new Key.fromObjectIdentity(this);
+  Key get key => new ObjectKey(this);
 }
 
 class EnsureVisibleApp extends App {

examples/widgets/overlay_geometry.dart

@@ -15,7 +15,7 @@ class CardModel {
   double height;
   Color color;
   String get label => "Card $value";
-  Key get key => new Key.fromObjectIdentity(this);
+  Key get key => new ObjectKey(this);
 }
 
 enum MarkerType { topLeft, bottomRight, touch }

examples/widgets/pageable_list.dart

@@ -12,7 +12,7 @@ class CardModel {
   Size size;
   Color color;
   String get label => "Card $value";
-  Key get key => new Key.fromObjectIdentity(this);
+  Key get key => new ObjectKey(this);
 }
 
 class PageableListApp extends App {

examples/widgets/tabs.dart

@@ -35,7 +35,7 @@ class TabbedNavigatorApp extends App {
           builder: () => _buildContent(text)
         );
       });
-    return _buildTabNavigator(n, views.toList(), const StringKey('textLabelsTabNavigator'));
+    return _buildTabNavigator(n, views.toList(), const ValueKey<String>('textLabelsTabNavigator'));
   }
 
   TabNavigator _buildIconLabelsTabNavigator(int n) {
@@ -46,7 +46,7 @@ class TabbedNavigatorApp extends App {
           builder: () => _buildContent(icon_name)
         );
       });
-    return _buildTabNavigator(n, views.toList(), const StringKey('iconLabelsTabNavigator'));
+    return _buildTabNavigator(n, views.toList(), const ValueKey<String>('iconLabelsTabNavigator'));
   }
 
   TabNavigator _buildTextAndIconLabelsTabNavigator(int n) {
@@ -64,7 +64,7 @@ class TabbedNavigatorApp extends App {
         builder: () => _buildContent("Summary")
       )
     ];
-    return _buildTabNavigator(n, views, const StringKey('textAndIconLabelsTabNavigator'));
+    return _buildTabNavigator(n, views, const ValueKey<String>('textAndIconLabelsTabNavigator'));
   }
 
   TabNavigator _buildScrollableTabNavigator(int n) {
@@ -86,7 +86,7 @@ class TabbedNavigatorApp extends App {
           builder: () => _buildContent(text)
         );
       });
-    return _buildTabNavigator(n, views.toList(), const StringKey('scrollableTabNavigator'), isScrollable: true);
+    return _buildTabNavigator(n, views.toList(), const ValueKey<String>('scrollableTabNavigator'), isScrollable: true);
   }
 
 
@@ -118,7 +118,7 @@ class TabbedNavigatorApp extends App {
       )
     ];
 
-    TabNavigator tabNavigator = _buildTabNavigator(4, views, const StringKey('tabs'));
+    TabNavigator tabNavigator = _buildTabNavigator(4, views, const ValueKey<String>('tabs'));
     assert(selectedIndices.length == 5);
 
     ToolBar toolbar = new ToolBar(

packages/flutter/lib/editing/input.dart

@@ -75,7 +75,7 @@ class Input extends StatefulComponent {
 
     if (placeholder != null && _value.isEmpty) {
       Widget child = new Opacity(
-        key: const StringKey('placeholder'),
+        key: const ValueKey<String>('placeholder'),
         child: new Text(placeholder, style: textStyle),
         opacity: themeData.hintOpacity
       );

packages/flutter/lib/widgets/basic.dart

@@ -768,7 +768,7 @@ class AssetImage extends Component {
 class WidgetToRenderBoxAdapter extends LeafRenderObjectWrapper {
   WidgetToRenderBoxAdapter(RenderBox renderBox)
     : renderBox = renderBox,
-      super(key: new Key.fromObjectIdentity(renderBox));
+      super(key: new ObjectKey(renderBox));
 
   final RenderBox renderBox;
 

packages/flutter/lib/widgets/dialog.dart

@@ -178,7 +178,7 @@ Future showDialog(Navigator navigator, DialogBuilder builder) {
     completer: completer,
     builder: (navigator, route) {
       return new Focus(
-        key: new GlobalKey.fromObjectIdentity(route),
+        key: new GlobalObjectKey(route),
         autofocus: true,
         child: builder(navigator)
       );

packages/flutter/lib/widgets/framework.dart

@@ -25,16 +25,14 @@ typedef void WidgetTreeWalker(Widget);
 
 abstract class Key {
   const Key.constructor(); // so that subclasses can call us, since the Key() factory constructor shadows the implicit constructor
-  factory Key(String value) => new StringKey(value);
-  factory Key.stringify(Object value) => new StringKey(value.toString());
-  factory Key.fromObjectIdentity(Object value) => new ObjectKey(value);
+  factory Key(String value) => new ValueKey<String>(value);
 }
 
-class StringKey extends Key {
-  const StringKey(this.value) : super.constructor();
-  final String value;
+class ValueKey<T> extends Key {
+  const ValueKey(this.value) : super.constructor();
+  final T value;
   String toString() => '[\'${value}\']';
-  bool operator==(other) => other is StringKey && other.value == value;
+  bool operator==(other) => other is ValueKey<T> && other.value == value;
   int get hashCode => value.hashCode;
 }
 
@@ -51,8 +49,7 @@ typedef void GlobalKeyRemoveListener(GlobalKey key);
 
 abstract class GlobalKey extends Key {
   const GlobalKey.constructor() : super.constructor(); // so that subclasses can call us, since the Key() factory constructor shadows the implicit constructor
-  factory GlobalKey({ String label }) => new LabeledGlobalKey(label);
-  factory GlobalKey.fromObjectIdentity(Object value) => new GlobalObjectKey(value);
+  factory GlobalKey({ String label }) => new LabeledGlobalKey(label); // the label is purely for debugging purposes and is otherwise ignored
 
   static final Map<GlobalKey, Widget> _registry = new Map<GlobalKey, Widget>();
   static final Map<GlobalKey, int> _debugDuplicates = new Map<GlobalKey, int>();

packages/flutter/lib/widgets/navigator.dart

@@ -186,7 +186,7 @@ class Navigator extends StatefulComponent {
       }
       if (child == null)
         continue;
-      TransitionBase transition = historyEntry.route.buildTransition(key: new Key.fromObjectIdentity(historyEntry))
+      TransitionBase transition = historyEntry.route.buildTransition(key: new ObjectKey(historyEntry))
         ..child = child
         ..direction = (i <= state.historyIndex) ? Direction.forward : Direction.reverse
         ..onDismissed = () {

packages/unit/test/widget/mimic_test.dart

@@ -13,7 +13,7 @@ void main() {
         new Mimicable(
           key: globalKey,
           child: new Container(
-            key: new Key.stringify('inner'),
+            key: new Key('inner'),
             height: 10.0,
             width: 10.0
           )
@@ -50,13 +50,13 @@ void main() {
     tester.pumpFrame(() {
       return new Flex([
         new Container(
-          key: new Key.stringify('outer'),
+          key: new Key('outer'),
           height: 10.0,
           width: 10.0,
           child: new Mimicable(
             key: globalKey,
             child: new Container(
-              key: new Key.stringify('inner'),
+              key: new Key('inner'),
               height: 10.0,
               width: 10.0
             )

packages/unit/test/widget/pageable_list_test.dart

@@ -14,7 +14,7 @@ void main() {
 
     Widget buildPage(int page) {
       return new Container(
-        key: new StringKey(page.toString()),
+        key: new ValueKey<int>(page),
         width: pageSize.width,
         height: pageSize.height,
         child: new Text(page.toString())