Add asserts to catch potential misuses of the rendering framework.

sky/engine/core/painting/Size.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.
 
 part of dart.sky;
 
 /// Holds a 2D floating-point size.
 /// Think of this as a vector from Point(0,0) to Point(size.width, size.height)
 class Size {
   const Size(this.width, this.height);
   Size.copy(Size source) : width = source.width, height = source.height;
   const Size.fromWidth(this.width) : height = double.INFINITY;
   const Size.fromHeight(this.height) : width = double.INFINITY;
 
   final double width;
   final double height;
 
   static const Size zero = const Size(0.0, 0.0);
   static const Size infinite = const Size(double.INFINITY, double.INFINITY);
 
   bool operator ==(other) => other is Size && width == other.width && height == other.height;
   Size operator +(Size other) => new Size(width + other.width, height + other.height);
   Size operator -(Size other) => new Size(width - other.width, height - other.height);
 
+  bool get isInfinite => width >= double.INFINITY || height >= double.INFINITY;
+
   // does the equivalent of "return new Point(0,0) + this"
   Point toPoint() => new Point(this.width, this.height);
 
   int get hashCode {
     int result = 373;
     result = 37 * result + width.hashCode;
     result = 37 * result + height.hashCode;
     return result;
   }
   String toString() => "Size($width, $height)";
 }