Added a key: parameter to ParentDataNode, ContentDataNode

The FlexExpandingChild flex parameter is now a keyword as
well, which adds a small readability benefit.

R=ianh@google.com

Review URL: https://codereview.chromium.org/1172503004.

examples/stocks2/lib/stock_app.dart

@@ -130,7 +130,7 @@ class StocksApp extends App {
           icon: 'action/thumb_up',
           onGestureTap: (event) => _handleStockModeChange(StockMode.optimistic),
           children: [
-            new FlexExpandingChild(new Text('Optimistic'), 1),
+            new FlexExpandingChild(new Text('Optimistic'), flex: 1),
             new Radio(key: 'optimistic-radio', value: StockMode.optimistic, groupValue: _stockMode, onChanged: _handleStockModeChange)
           ]),
         new MenuItem(
@@ -138,7 +138,7 @@ class StocksApp extends App {
           icon: 'action/thumb_down',
           onGestureTap: (event) => _handleStockModeChange(StockMode.pessimistic),
           children: [
-            new FlexExpandingChild(new Text('Pessimistic'), 1),
+            new FlexExpandingChild(new Text('Pessimistic'), flex: 1),
             new Radio(key: 'pessimistic-radio', value: StockMode.pessimistic, groupValue: _stockMode, onChanged: _handleStockModeChange)
           ]),
         new MenuDivider(key: 'div2'),

sdk/lib/framework/components2/menu_item.dart

@@ -11,7 +11,7 @@ class MenuItem extends ButtonBase {
 
   static const BoxDecoration highlightDecoration = const BoxDecoration(
     backgroundColor: const Color.fromARGB(102, 153, 153, 153)
-  ); 
+  );
 
   List<UINode> children;
   String icon;
@@ -36,7 +36,7 @@ class MenuItem extends ButtonBase {
                 ),
                 padding: const EdgeDims.symmetric(horizontal: 16.0)
               ),
-              1
+              flex: 1
             )
           ]
         ),

sdk/lib/framework/fn2.dart

@@ -154,7 +154,7 @@ abstract class UINode {
 abstract class ContentNode extends UINode {
   UINode content;
 
-  ContentNode(UINode content) : this.content = content, super(key: content._key);
+  ContentNode(UINode content, { Object key }) : this.content = content, super(key: key);
 
   void _sync(UINode old, dynamic slot) {
     UINode oldContent = old == null ? null : (old as ContentNode).content;
@@ -173,7 +173,7 @@ abstract class ContentNode extends UINode {
 class ParentDataNode extends ContentNode {
   final ParentData parentData;
 
-  ParentDataNode(UINode content, this.parentData): super(content);
+  ParentDataNode(UINode content, this.parentData, { Object key }): super(content, key: key);
 }
 
 typedef void GestureEventListener(sky.GestureEvent e);
@@ -754,8 +754,8 @@ class FlexContainer extends MultiChildRenderObjectWrapper {
 }
 
 class FlexExpandingChild extends ParentDataNode {
-  FlexExpandingChild(UINode content, [int flex = 1])
-    : super(content, new FlexBoxParentData()..flex = flex);
+  FlexExpandingChild(UINode content, { int flex: 1, Object key })
+    : super(content, new FlexBoxParentData()..flex = flex, key: key);
 }
 
 class Image extends RenderObjectWrapper {