Introduce RenderProxyBox and RenderSizedBox

Also makes Point, Size, and Rect immutable.

R=ianh@google.com

Review URL: https://codereview.chromium.org/1162033002

examples/raw/sector-layout.dart

@@ -126,10 +126,10 @@ class RenderDecoratedSector extends RenderSector {
       sky.Paint paint = new sky.Paint()..color = _decoration.backgroundColor;
       sky.Path path = new sky.Path();
       double outerRadius = (parentData.radius + deltaRadius);
-      sky.Rect outerBounds = new sky.Rect()..setLTRB(-outerRadius, -outerRadius, outerRadius, outerRadius);
+      sky.Rect outerBounds = new sky.Rect.fromLTRB(-outerRadius, -outerRadius, outerRadius, outerRadius);
       path.arcTo(outerBounds, deg(parentData.theta), deg(deltaTheta), true);
       double innerRadius = parentData.radius;
-      sky.Rect innerBounds = new sky.Rect()..setLTRB(-innerRadius, -innerRadius, innerRadius, innerRadius);
+      sky.Rect innerBounds = new sky.Rect.fromLTRB(-innerRadius, -innerRadius, innerRadius, innerRadius);
       path.arcTo(innerBounds, deg(parentData.theta + deltaTheta), deg(-deltaTheta), false);
       path.close();
       canvas.drawPath(path, paint);

examples/raw/simple_render_tree.dart

@@ -10,7 +10,7 @@ class RenderSolidColor extends RenderDecoratedBox {
   final Size desiredSize;
   final int backgroundColor;
 
-  RenderSolidColor(int backgroundColor, { this.desiredSize })
+  RenderSolidColor(int backgroundColor, { this.desiredSize: const sky.Size.infinite() })
       : backgroundColor = backgroundColor,
         super(new BoxDecoration(backgroundColor: backgroundColor)) {
   }
@@ -41,7 +41,7 @@ void main() {
       decoration: new BoxDecoration(backgroundColor: 0xFF000000));
 
   void addFlexChild(RenderFlex parent, int backgroundColor, { int flex: 0 }) {
-    RenderNode child = new RenderSolidColor(backgroundColor, desiredSize: new Size.infinite());
+    RenderNode child = new RenderSolidColor(backgroundColor);
     parent.add(child);
     child.parentData.flex = flex;
   }

examples/raw/spinning_square.dart

@@ -13,7 +13,7 @@ void beginFrame(double timeStamp) {
   PictureRecorder canvas = new PictureRecorder(view.width, view.height);
   canvas.translate(view.width / 2.0, view.height / 2.0);
   canvas.rotateDegrees(delta / 10);
-  canvas.drawRect(new Rect()..setLTRB(-100.0, -100.0, 100.0, 100.0),
+  canvas.drawRect(new Rect.fromLTRB(-100.0, -100.0, 100.0, 100.0),
                   new Paint()..setARGB(255, 0, 255, 0));
   view.picture = canvas.endRecording();
   view.scheduleFrame();

examples/spinning_arabic.dart

@@ -15,7 +15,7 @@ void beginFrame(double timeStamp) {
   PictureRecorder canvas = new PictureRecorder(view.width, view.height);
   canvas.translate(view.width / 2.0, view.height / 2.0);
   canvas.rotateDegrees(delta / 10);
-  canvas.drawRect(new Rect()..setLTRB(-100.0, -100.0, 100.0, 100.0),
+  canvas.drawRect(new Rect.fromLTRB(-100.0, -100.0, 100.0, 100.0),
                   new Paint()..setARGB(255, 0, 255, 0));
 
   double sin = math.sin(delta / 200);