Make changing themes work again.

sky/sdk/lib/widgets/README.md

 Sky Widgets
 ===========
 
 Sky widgets are built using a functional-reactive framework, which takes
 inspiration from [React](http://facebook.github.io/react/). The central idea is
 that you build your UI out of components. Components describe what their view
 should look like given their current configuration and state. When a component's
 state changes, the component rebuilds its description, which the framework diffs
 against the previous description in order to determine the minial changes needed
 in the underlying render tree to transition from one state to the next.
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    source of those values.
 
    By convention, components typically store the values they receive from their
    parents in public member variables and their own internal state in private
    member variables. Therefore, a typical `syncFields` implementation will copy
    the public, but not the private, member variables from `source`. When
    following this convention, there is no need to copy over the private member
    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.
 
 didMount and didUnmount
 -----------------------
 
 When a component is inserted into the widget tree, the framework calls the
 `didMount` function on the component. When a component is removed from the
 widget tree, the framework calls the `didUnmount` function on the component.
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    first entry in the previous build, even if, semantically, the first entry in
    the list just scrolled off screen and is no longer visible in the viewport.
 
  * By assigning each entry in the list a "semantic" key, the infinite list can
    be more efficient because the framework will sync entries with matching
    semantic keys and therefore similiar (or identical) visual appearances.
    Moreover, syncing the entries semantically means that state retained in
    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
 ------------
 
  * `package:vector_math`
  * [`package:sky/animation`](../animation)
  * [`package:sky/base`](../base)
  * [`package:sky/painting`](../painting)
  * [`package:sky/rendering`](../rendering)
  * [`package:sky/theme`](../theme)