Make RenderParagraph mutable, and make it fit the new RenderNode protocols

sky/tests/raw/render_box.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 '../resources/third_party/unittest/unittest.dart';
 import '../resources/unit.dart';
 import 'dart:sky' as sky;
 import 'package:sky/framework/layout2.dart';
 
 void main() {
   initUnit();
 
   test("should size to render view", () {
     RenderSizedBox root = new RenderSizedBox(
-        new RenderDecoratedBox(new BoxDecoration(backgroundColor: 0xFF00FF00)));
+      child: new RenderDecoratedBox(
+        decoration: new BoxDecoration(backgroundColor: 0xFF00FF00)
+      )
+    );
     RenderView renderView = new RenderView(child: root);
     renderView.layout(new ViewConstraints(width: sky.view.width, height: sky.view.height));
     expect(root.size.width, equals(sky.view.width));
     expect(root.size.height, equals(sky.view.height));
   });
 }