Bring back initPolymer, allow boot.js only if using "polymer_experimental.html".

pkg/polymer/test/prop_attr_bind_reflection_test.dart

Index: pkg/polymer/test/prop_attr_bind_reflection_test.dart
diff --git a/pkg/polymer/test/prop_attr_bind_reflection_test.dart b/pkg/polymer/test/prop_attr_bind_reflection_test.dart
index ce064748a0437bc8e21d8934ed5cf889d140d48c..29caabd2aa84494ad380a4cd1ca341706fea80be 100644
--- a/pkg/polymer/test/prop_attr_bind_reflection_test.dart
+++ b/pkg/polymer/test/prop_attr_bind_reflection_test.dart
@@ -29,8 +29,7 @@ class MyElement extends PolymerElement {
   MyElement.created() : super.created();
 }
 
-@initMethod
-main() {
+main() => initPolymer().run(() {
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);
@@ -44,4 +43,4 @@ main() {
     expect('11', child.attributes['lowercase']);
     expect('11', child.attributes['camelcase']);
   });
-}
+});