Introduce boot.js: this finally makes it possible to load and run Todomvc

lib/observe_html.dart

Index: lib/observe_html.dart
diff --git a/lib/observe_html.dart b/lib/observe_html.dart
index 719655ddebfd1c5559de587cfeb3dc63a7b160b2..bae2fb2d85c5882fc3995c75b653e84d74438920 100644
--- a/lib/observe_html.dart
+++ b/lib/observe_html.dart
@@ -15,7 +15,7 @@ import 'package:observe/observe.dart';
 /** An observable version of [window.location.hash]. */
 final ObservableLocationHash windowLocation = new ObservableLocationHash._();
 
-class ObservableLocationHash extends ObservableBase {
+class ObservableLocationHash extends ChangeNotifierBase {
   ObservableLocationHash._() {
     // listen on changes to #hash in the URL
     // Note: listen on both popState and hashChange, because IE9 doesn't support