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

pkg/polymer/test/instance_attrs_test.dart

Index: pkg/polymer/test/instance_attrs_test.dart
diff --git a/pkg/polymer/test/instance_attrs_test.dart b/pkg/polymer/test/instance_attrs_test.dart
index f5b6eac7b9d4c3cb677a5da3990e8a05784b6b59..24f94cbb05e782157bade0a3eca0e652e818927d 100644
--- a/pkg/polymer/test/instance_attrs_test.dart
+++ b/pkg/polymer/test/instance_attrs_test.dart
@@ -17,8 +17,8 @@ class MyElement extends PolymerElement {
   get attributes => super.attributes;
 }
 
+@initMethod
 main() {
-  initPolymer();
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);