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

sky/sdk/lib/widgets/tabs.dart

Index: sky/sdk/lib/widgets/tabs.dart
diff --git a/sky/sdk/lib/widgets/tabs.dart b/sky/sdk/lib/widgets/tabs.dart
index 198e1b4818bd5f33b1e20e489da4a09d58208c3a..9d8af98fa98666f5fb7fbc5ccdc51cf15857e544 100644
--- a/sky/sdk/lib/widgets/tabs.dart
+++ b/sky/sdk/lib/widgets/tabs.dart
@@ -94,8 +94,7 @@ class RenderTabBar extends RenderBox with
     assert(constraints is BoxConstraints);
 
     size = constraints.constrain(new Size(constraints.maxWidth, _kTabBarHeight));
-    assert(size.width < double.INFINITY);
-    assert(size.height < double.INFINITY);
+    assert(!size.isInfinite);
 
     int childCount = _childCount();
     if (childCount == 0)