Actually perform a block layout

sky/examples/raw/simple_render_tree.dart

Index: sky/examples/raw/simple_render_tree.dart
diff --git a/sky/examples/raw/simple_render_tree.dart b/sky/examples/raw/simple_render_tree.dart
index f5ae7e7dc0e52f106ec8a1c8cafc68ef4efb3371..f388c0744bc2101a24215f1b168df3331fb44dcb 100644
--- a/sky/examples/raw/simple_render_tree.dart
+++ b/sky/examples/raw/simple_render_tree.dart
@@ -6,16 +6,31 @@ import 'dart:math';
 import 'dart:sky';
 import 'package:sky/framework/layout2.dart';
 
-class RenderBlueCircle extends RenderBox {
-  void paint(RenderNodeDisplayList canvas) {
-    double radius = min(width, height) * 0.45;
-    Paint paint = new Paint()..setARGB(255, 0, 255, 255);
-    canvas.drawCircle(width / 2, height / 2, radius, paint);
+class SimpleBlock extends RenderDecoratedBox {
+  SimpleBlock(int backgroundColor)
+      : super(new BoxDecoration(backgroundColor: backgroundColor));
+
+  BoxDimensions getIntrinsicDimensions(BoxConstraints constraints) {
+    return new BoxDimensions.withConstraints(constraints, height: 200.0);
+  }
+
+  void layout(BoxConstraints constraints, { RenderNode relayoutSubtreeRoot }) {
+    width = clamp(min: constraints.minWidth,
+                  max: constraints.maxWidth, value: constraints.maxWidth);
+    height = clamp(min: constraints.minHeight,
+                   max: constraints.maxHeight, value: 200.0);
+    layoutDone();
   }
 }
 
 void main() {
-  RenderView renderView = new RenderView(root: new RenderBlueCircle());
+  var root = new RenderBlock(
+      decoration: new BoxDecoration(backgroundColor: 0xFF00FFFF));
+
+  root.add(new SimpleBlock(0xFF00FF00));
+  root.add(new SimpleBlock(0xFF0000FF));
+
+  RenderView renderView = new RenderView(root: root);
   renderView.layout(newWidth: view.width, newHeight: view.height);
   renderView.paintFrame();
 }