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

pkg/polymer/test/custom_event_test.dart

Index: pkg/polymer/test/custom_event_test.dart
diff --git a/pkg/polymer/test/custom_event_test.dart b/pkg/polymer/test/custom_event_test.dart
index b5840294848533777bec13dbac28def0c4a70441..0b67e3e10eb1b1dcd4ec8d1bb43d458d5e2ab1af 100644
--- a/pkg/polymer/test/custom_event_test.dart
+++ b/pkg/polymer/test/custom_event_test.dart
@@ -42,8 +42,8 @@ class TestCustomEvent extends PolymerElement {
   barBazHandler(e) => events.add(['barbaz', e]);
 }
 
+@initMethod
 main() {
-  initPolymer();
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);