Replace bootstrap logic with 'boot.js', use 'component/dart' mime-type and add

pkg/polymer/test/event_path_test.dart

Index: pkg/polymer/test/event_path_test.dart
diff --git a/pkg/polymer/test/event_path_test.dart b/pkg/polymer/test/event_path_test.dart
index 121b346788535065ad34f5db76971f74ebd24501..ecf922f5ce9955d4bf8f475e2f0a622a77289de1 100644
--- a/pkg/polymer/test/event_path_test.dart
+++ b/pkg/polymer/test/event_path_test.dart
@@ -25,6 +25,7 @@ class XMenuButton extends PolymerElement {
   XMenuButton.created() : super.created();
 }
 
+@initMethod
 main() {
   // TODO(sigmund): use @CustomTag instead of Polymer.regsiter. A bug is making
   // this code sensitive to the order in which we register elements (e.g. if
@@ -34,7 +35,6 @@ main() {
   Polymer.register('x-menu', XMenu);
   Polymer.register('x-menu-button', XMenuButton);
 
-  initPolymer();
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);