Refactor stateful parts of Component into StatefulComponent

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.
@@ skipping 236 matching lines @@
       onGestureTap: (_) => onChanged(!value),
       child: new Container(
         height: 25.0,
         width: 25.0,
         decoration: new BoxDecoration(backgroundColor: color)
       )
     );
   }
 }
 
-class MyDialog extends Component {
-  MyDialog({ this.onDismissed }) : super(stateful: true);
+class MyDialog extends StatefulComponent {
+  MyDialog({ this.onDismissed });
 
   Function onDismissed;
   bool _checkboxValue = false;
 
   void _handleCheckboxValueChanged(bool value) {
     setState(() {
       _checkboxValue = value;
     });
   }
 
@@ skipping 22 matching lines @@
 which it then uses during its `build` function. Notice that when the user taps
 on the checkbox, the checkbox itself doesn't use `value`. Instead, the checkbox
 calls a function it received from its parent component. This pattern lets you
 store state higher in the component hierarchy, which causes the state to persist
 for longer periods of time. In the extreme, the state stored on the `App`
 component persists for the lifetime of the application.
 
 The `MyDialog` component is more complicated because it is a stateful component.
 Let's walk through the differences in `MyDialog` caused by its being stateful:
 
- * `MyDialog` passes `stateful: true` to the `Component` constructor, marking
-   the component stateful.
+ * `MyDialog` extends StatefulComponent instead of Component.
 
  * `MyDialog` has non-`final` member variables. Over the lifetime of the dialog,
    we'll need to modify the values of these member variables, which means we
    cannot mark them `final`.
 
  * `MyDialog` has private member variables. By convention, components store
    values they receive from their parent in public member variables and store
    their own internal, transient state in private member variables. There's no
    requirement to follow this convention, but we've found that it helps keep us
    organized.
@@ skipping 101 matching lines @@
 
 Dependencies
 ------------
 
  * `package:vector_math`
  * [`package:sky/animation`](../animation)
  * [`package:sky/base`](../base)
  * [`package:sky/painting`](../painting)
  * [`package:sky/rendering`](../rendering)
  * [`package:sky/theme`](../theme)