Make changing themes work again.

sky/sdk/lib/rendering/README.md

 Sky Rendering
 =============
 
 The Sky render tree is a low-level layout and painting system based on a
 retained tree of objects that inherit from [`RenderObject`](object.dart). Most
 developers using Sky will not need to interact directly with the rendering tree.
 Instead, most developers should use [Sky widgets](../widgets/README.md), which
 are built using the render tree.
 
 Overview
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 Performance rules of thumb
 --------------------------
 
 * Avoid using transforms where mere maths would be sufficient (e.g.
   draw your rectangle at x,y rather than translating by x,y and
   drawing it at 0,0).
 
 * Avoid using save/restore on canvases.
 
 
+Useful debugging tools
+----------------------
+
+This is a quick way to dump the entire render tree to the console.
+This can be quite useful in figuring out exactly what is going on when
+working with the render tree.
+
+```dart
+import 'package:sky/rendering/sky_binding.dart';
+
+SkyBinding.instance.debugDumpRenderTree();
+```
+
+
 Dependencies
 ------------
 
  * [`package:sky/base`](../base)
  * [`package:sky/mojo`](../mojo)
  * [`package:sky/animation`](../mojo)