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

sky/examples/raw/sector-layout.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:math' as math;
 import 'dart:sky' as sky;
 import 'package:sky/framework/app.dart';
 import 'package:sky/framework/layout2.dart';
 
 const double kTwoPi = 2 * math.PI;
@@ skipping 398 matching lines @@
     double dimension = (innerRadius + childDimensions.deltaRadius) * 2.0;
     return constraints.constrain(new sky.Size(dimension, dimension));
   }
 
   void performLayout() {
     if (child == null) {
       size = constraints.constrain(new sky.Size(0.0, 0.0));
     } else {
       assert(child is RenderSector);
       assert(!constraints.isInfinite);
+      print("constraint maxes: ${constraints.maxWidth} and ${constraints.maxHeight}");
       double maxChildDeltaRadius = math.min(constraints.maxWidth, constraints.maxHeight) / 2.0 - innerRadius;
+      print("maxChildDeltaRadius = $maxChildDeltaRadius");
       assert(child.parentData is SectorParentData);
       child.parentData.radius = innerRadius;
       child.parentData.theta = 0.0;
       child.layout(new SectorConstraints(maxDeltaRadius: maxChildDeltaRadius), parentUsesSize: true);
       double dimension = (innerRadius + child.deltaRadius) * 2.0;
       size = constraints.constrain(new sky.Size(dimension, dimension));
     }
   }
 
   // paint origin is 0,0 of our circle
@@ skipping 66 matching lines @@
   rootCircle.add(new RenderSolidColor(0xFF0000FF, desiredDeltaTheta: kTwoPi * 0.4));
   var stack = new RenderSectorSlice(padding: 2.0);
   stack.add(new RenderSolidColor(0xFFFFFF00, desiredDeltaRadius: 20.0));
   stack.add(new RenderSolidColor(0xFFFF9000, desiredDeltaRadius: 20.0));
   stack.add(new RenderSolidColor(0xFF00FF00));
   rootCircle.add(stack);
 
   var root = new RenderBoxToRenderSectorAdapter(innerRadius: 50.0, child: rootCircle);
   app = new AppView(root);
 }