Add support for transforms to container

These transforms are currently very basic but they seem to mostly work.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1152273004

examples/fn2/container.dart

@@ -10,7 +10,6 @@ class ContainerApp extends App {
   UINode build() {
     return new EventListenerNode(
       new BlockContainer(children: [
-        new Rectangle(0xFF00FFFF, key: 1),
         new Container(
           padding: new EdgeDims.all(10.0),
           margin: new EdgeDims.all(10.0),
@@ -23,7 +22,6 @@ class ContainerApp extends App {
                       desiredSize: new sky.Size(double.INFINITY, 20.0)
                   )
               ])),
-        new Rectangle(0xFF0000FF, key: 3)
       ]),
       onPointerDown: _handlePointerDown);
   }

examples/raw/transform.dart

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:sky' as sky;
+import 'package:sky/framework/app.dart';
+import 'package:sky/framework/rendering/flex.dart';
+import 'package:sky/framework/rendering/box.dart';
+import 'package:vector_math/vector_math.dart';
+
+AppView app;
+
+void main() {
+  RenderDecoratedBox green = new RenderDecoratedBox(
+      decoration: new BoxDecoration(backgroundColor: 0xFF00FF00));
+  RenderSizedBox box = new RenderSizedBox(
+      desiredSize: new sky.Size(200.0, 200.0), child: green);
+
+  Matrix4 transform = new Matrix4.identity();
+  RenderTransform spin = new RenderTransform(
+      transform: transform, child: box);
+  spin.rotateZ(1.0);
+
+  RenderFlex flex = new RenderFlex();
+  flex.add(spin);
+  app = new AppView(flex);
+}