Make changing themes work again.

sky/sdk/lib/widgets/README.md

Index: sky/sdk/lib/widgets/README.md
diff --git a/sky/sdk/lib/widgets/README.md b/sky/sdk/lib/widgets/README.md
index 478b64393227c545f2cf1a9711d7e3d824bd5c81..70df5f226cfcf46509e01b4c3023defa2c4dd3dc 100644
--- a/sky/sdk/lib/widgets/README.md
+++ b/sky/sdk/lib/widgets/README.md
@@ -355,6 +355,10 @@ Let's walk through the differences in `MyDialog` caused by its being stateful:
    variables because those represent the internal state of the object and `this`
    is the authoritative source of that state.
 
+   When implementing a `StatefulComponent`, make sure to call
+   `super.syncFields(source)` from within your `syncFields()` method,
+   unless you are extending `StatefulComponent` directly.
+
 Finally, when the user taps on the "Save" button, `MyDialog` follows the same
 pattern as `MyCheckbox` and calls a function passed in by its parent component
 to return the final value of the checkbox up the hierarchy.
@@ -427,6 +431,20 @@ copies of a particular widget to fill its visible region:
    stateful subcomponents will remain attached to the same semantic entry rather
    than the entry in the same numerical position in the viewport.
 
+Useful debugging tools
+----------------------
+
+This is a quick way to dump the entire widget tree to the console.
+This can be quite useful in figuring out exactly what is going on when
+working with the widgets system. For this to work, you have to have
+launched your app with `runApp()`.
+
+```dart
+import 'package:sky/widget/widget.dart';
+
+debugDumpApp();
+```
+
 Dependencies
 ------------