Let's hide double.INFINITY a bit more, by providing cleaner APIs for the cases where we're currentl…

sky/sdk/lib/rendering/block.dart

Index: sky/sdk/lib/rendering/block.dart
diff --git a/sky/sdk/lib/rendering/block.dart b/sky/sdk/lib/rendering/block.dart
index 3ab31711dfecd20e391903a295207dca87d4cdf8..fb5b7c6e5a083b34bf02543bd47634ca7a02f9a5 100644
--- a/sky/sdk/lib/rendering/block.dart
+++ b/sky/sdk/lib/rendering/block.dart
@@ -96,8 +96,7 @@ class RenderBlock extends RenderBox with ContainerRenderObjectMixin<RenderBox, B
       child = child.parentData.nextSibling;
     }
     size = new Size(width, constraints.constrainHeight(y));
-    assert(size.width < double.INFINITY);
-    assert(size.height < double.INFINITY);
+    assert(!size.isInfinite);
   }
 
   void hitTestChildren(HitTestResult result, { Point position }) {