feat(change detect): Throw on attempts to use dehydrated detector

- Modify change detectors to `throw` when attempting to detect changes
  on a dehydrated detector.
- Modify `DynamicChagneDetector` to use `null` for the `context` of a
  dehydrated detector.

modules/angular2/change_detection.ts

@@ -19,6 +19,7 @@ export {Parser} from './src/change_detection/parser/parser';
 export {Locals} from './src/change_detection/parser/locals';
 
 export {
+  DehydratedException,
   ExpressionChangedAfterItHasBeenChecked,
   ChangeDetectionError
 } from './src/change_detection/exceptions';