Give ParentDataNodes snappier names

FlexExpandingChild -> Flexible
StackPositionedChild -> Positioned

Also, name the child argument |child| for consistency.

TBR=ianh@google.com

Review URL: https://codereview.chromium.org/1179763009.

examples/stocks2/lib/stock_app.dart

@@ -127,7 +127,7 @@ class StocksApp extends App {
           icon: 'action/thumb_up',
           onGestureTap: (event) => _handleStockModeChange(StockMode.optimistic),
           children: [
-            new FlexExpandingChild(new Text('Optimistic'), flex: 1),
+            new Flexible(child: new Text('Optimistic')),
             new Radio(key: 'optimistic-radio', value: StockMode.optimistic, groupValue: _stockMode, onChanged: _handleStockModeChange)
           ]),
         new MenuItem(
@@ -135,7 +135,7 @@ class StocksApp extends App {
           icon: 'action/thumb_down',
           onGestureTap: (event) => _handleStockModeChange(StockMode.pessimistic),
           children: [
-            new FlexExpandingChild(new Text('Pessimistic'), flex: 1),
+            new Flexible(child: new Text('Pessimistic')),
             new Radio(key: 'pessimistic-radio', value: StockMode.pessimistic, groupValue: _stockMode, onChanged: _handleStockModeChange)
           ]),
         new MenuDivider(key: 'div2'),

examples/stocks2/lib/stock_menu.dart

@@ -26,13 +26,13 @@ class StockMenu extends Component {
       onChanged: this.onAutorefreshChanged
     );
 
-    return new StackPositionedChild(
-      new PopupMenu(
+    return new Positioned(
+      child: new PopupMenu(
         controller: controller,
         items: [
           [new Text('Add stock')],
           [new Text('Remove stock')],
-          [new FlexExpandingChild(new Text('Autorefresh')), checkbox],
+          [new Flexible(child: new Text('Autorefresh')), checkbox],
         ],
         level: 4
       ),

examples/stocks2/lib/stock_row.dart

@@ -29,12 +29,12 @@ class StockRow extends Component {
       new Container(
           child: new StockArrow(percentChange: stock.percentChange),
           margin: const EdgeDims.only(right: 5.0)),
-      new FlexExpandingChild(new Text(stock.symbol), flex: 2, key: "symbol"),
+      new Flexible(child: new Text(stock.symbol), flex: 2, key: "symbol"),
       // TODO(hansmuller): text-align: right
-      new FlexExpandingChild(new Text(lastSale,
+      new Flexible(child: new Text(lastSale,
             style: const TextStyle(textAlign: TextAlign.right)),
           key: "lastSale"),
-      new FlexExpandingChild(new Text(changeInPrice,
+      new Flexible(child: new Text(changeInPrice,
           style: typography.black.caption.copyWith(textAlign: TextAlign.right)),
         key: "changeInPrice")
     ];

examples/widgets/container.dart

@@ -38,8 +38,8 @@ class ContainerApp extends App {
             )
           ])
         ),
-        new FlexExpandingChild(
-          new Container(
+        new Flexible(
+          child: new Container(
             decoration: new BoxDecoration(backgroundColor: const Color(0xFF00FFFF))
           )
         ),

examples/widgets/spinning_mixed.dart

@@ -27,8 +27,8 @@ class Rectangle extends Component {
   Rectangle(this.color, { Object key }) : super(key: key);
   final Color color;
   UINode build() {
-    return new FlexExpandingChild(
-      new Container(
+    return new Flexible(
+      child: new Container(
         decoration: new BoxDecoration(backgroundColor: color)
       )
     );