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

pkg/polymer/test/take_attributes_test.dart

Index: pkg/polymer/test/take_attributes_test.dart
diff --git a/pkg/polymer/test/take_attributes_test.dart b/pkg/polymer/test/take_attributes_test.dart
index 8267debb3c1cc64fdd3ecd6a6da3dd320e3f2d69..c77a1a4e4c5b6cecdd022958cda87d31c60ef80a 100644
--- a/pkg/polymer/test/take_attributes_test.dart
+++ b/pkg/polymer/test/take_attributes_test.dart
@@ -49,8 +49,8 @@ class XObj extends PolymerElement {
   @observable var values = {};
 }
 
+@initMethod
 main() {
-  initPolymer();
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);