Add a simple_render_tree example

sky/sdk/lib/framework/layout2.dart

 library layout;
 
 // This version of layout.dart is an update to the other one, this one using new APIs.
 // It will not work in a stock Sky setup currently.
 
 import 'node.dart';
 
 import 'dart:sky' as sky;
 
 // ABSTRACT LAYOUT
@@ skipping 210 matching lines @@
 
 
   // PAINTING
 
   static bool _debugDoingPaint = false;
   void markNeedsPaint() {
     assert(!_debugDoingPaint);
     var ancestor = this;
     while (ancestor.parent != null)
       ancestor = ancestor.parent;
-    assert(ancestor is Screen);
+    assert(ancestor is RenderView);
     ancestor.paintFrame();
   }
   void paint(RenderNodeDisplayList canvas) { }
 
 }
 
 
 // GENERIC MIXIN FOR RENDER NODES THAT TAKE A LIST OF CHILDREN
 
 abstract class ContainerParentDataMixin<ChildType extends RenderNode> {
@@ skipping 201 matching lines @@
 
   void rotate({
     int oldAngle, // 0..3
     int newAngle, // 0..3
     Duration time
   }) { }
 
 }
 
 
-// SCREEN LAYOUT MANAGER
+// RENDER VIEW LAYOUT MANAGER
 
-class Screen extends RenderNode {
+class RenderView extends RenderNode {
 
-  Screen({
+  RenderView({
     RenderBox root,
     this.timeForRotation: const Duration(microseconds: 83333)
   }) {
     assert(root != null);
     this.root = root;
   }
 
   double _width;
   double get width => _width;
   double _height;
   double get height => _height;
 
   int _orientation; // 0..3
   int get orientation => _orientation;
   Duration timeForRotation;
 
   RenderBox _root;
   RenderBox get root => _root;
   void set root (RenderBox value) {
-    assert(root != null);
+    assert(value != null);
     _root = value;
     adoptChild(_root);
     markNeedsLayout();
   }
 
   void layout({
     double newWidth,
     double newHeight,
     int newOrientation
   }) {
@@ skipping 52 matching lines @@
   final double bottom;
   final double left;
   operator ==(EdgeDims other) => (top == other.top) ||
                                  (right == other.right) ||
                                  (bottom == other.bottom) ||
                                  (left == other.left);
 }
 
 class BlockParentData extends BoxParentData with ContainerParentDataMixin<RenderBox> { }
 
-class BlockBox extends RenderBox with ContainerRenderNodeMixin<RenderBox, BlockParentData> {
+class RenderBlock extends RenderBox with ContainerRenderNodeMixin<RenderBox, BlockParentData> {
   // lays out RenderBox children in a vertical stack
   // uses the maximum width provided by the parent
   // sizes itself to the height of its child stack
 
-  BlockBox({
+  RenderBlock({
     EdgeDims padding: const EdgeDims(0.0, 0.0, 0.0, 0.0)
   }) {
     _padding = padding;
   }
 
   EdgeDims _padding;
   EdgeDims get padding => _padding;
   void set padding(EdgeDims value) {
     assert(value != null);
     if (_padding != value) {
@@ skipping 182 matching lines @@
     canvas.paintChild(body, (body.parentData as BoxParentData).x, (body.parentData as BoxParentData).y);
     if (statusbar != null)
       canvas.paintChild(statusbar, (statusbar.parentData as BoxParentData).x, (statusbar.parentData as BoxParentData).y);
     if (toolbar != null)
       canvas.paintChild(toolbar, (toolbar.parentData as BoxParentData).x, (toolbar.parentData as BoxParentData).y);
     if (drawer != null)
       canvas.paintChild(drawer, (drawer.parentData as BoxParentData).x, (drawer.parentData as BoxParentData).y);
   }
 
 }