Rename BlockContainer, StackContainer, and FlexContainer

These are now called Block, Stack, and Flex, respectively. Less verbose.

R=jackson@google.com, ianh@google.com

Review URL: https://codereview.chromium.org/1181533002.

examples/fn2/container.dart

@@ -20,14 +20,13 @@ class Rectangle extends RenderObjectWrapper {
 class ContainerApp extends App {
   UINode build() {
     return new EventListenerNode(
-      new BlockContainer(children: [
+      new Block([
         new Container(
           padding: new EdgeDims.all(10.0),
           margin: new EdgeDims.all(10.0),
           height: 100.0,
           decoration: new BoxDecoration(backgroundColor: const sky.Color(0xFF00FF00)),
-          child: new BlockContainer(
-              children: [
+          child: new Block([
             new Container(
               decoration: new BoxDecoration(backgroundColor: const sky.Color(0xFFFFFF00)),
               height: 20.0,

examples/stocks2/lib/stock_app.dart

@@ -212,7 +212,7 @@ class StocksApp extends App {
       ),
     ];
     addMenuToOverlays(overlays);
-    return new StackContainer(children: overlays);
+    return new Stack(overlays);
   }
 }
 

examples/stocks2/lib/stock_row.dart

@@ -44,7 +44,7 @@ class StockRow extends Component {
         decoration: const BoxDecoration(
             border: const Border(
                 bottom: const BorderSide(color: const sky.Color(0xFFF4F4F4)))),
-        child: new FlexContainer(children: children)
+        child: new Flex(children)
       )
     ]);
   }