Get rid of some hanging js.Proxy's and the corresponding release() methods on
Window and Tab.

Change Runtime.onMessage to a Stream<MessageEvent> and add test.

Change ChromeStreamController to automatically add and remove js listeners
as Dart listeners are added and removed.

Change background page to have functionality implemented in Dart.
More cleanup and fixes.
Merge branch 'master' of https://github.com/dart-gde/chrome.dart

Conflicts:
	test/interop.js
	tool/hop_runner.dart
Remove const from context_menus.dart.
Merge pull request #68 from DrMarcII/master

Cleanup of extension and application APIs