Add BlockContainer to fn2

BlockContainer uses a RenderBlock to lay out its children.

R=eseidel@chromium.org, jackson@chromium.org

Review URL: https://codereview.chromium.org/1166473005

examples/fn2/container.dart

@@ -8,7 +8,11 @@ import 'package:sky/framework/fn2.dart';
 class ContainerApp extends App {
   UINode build() {
     return new EventListenerNode(
-      new Rectangle(0xFF00FFFF),
+      new BlockContainer(children: [
+        new Rectangle(0xFF00FFFF, key: 1),
+        new Rectangle(0xFF00FF00, key: 2),
+        new Rectangle(0xFF0000FF, key: 3)
+      ]),
       onPointerDown: _handlePointerDown);
   }
 

sdk/lib/framework/fn2.dart

@@ -501,16 +501,16 @@ abstract class OneChildListRenderNodeWrapper extends RenderNodeWrapper {
   }
 }
 
-class Container extends OneChildListRenderNodeWrapper {
+class BlockContainer extends OneChildListRenderNodeWrapper {
 
-  RenderCSSContainer root;
-  RenderCSSContainer createNode() => new RenderCSSContainer(this);
+  RenderBlock root;
+  RenderBlock createNode() => new RenderBlock();
 
-  static final Container _emptyContainer = new Container();
+  static final BlockContainer _emptyContainer = new BlockContainer();
 
   RenderNodeWrapper get emptyNode => _emptyContainer;
 
-  Container({
+  BlockContainer({
     Object key,
     List<UINode> children
   }) : super(