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

pkg/polymer/test/prop_attr_reflection_test.dart

Index: pkg/polymer/test/prop_attr_reflection_test.dart
diff --git a/pkg/polymer/test/prop_attr_reflection_test.dart b/pkg/polymer/test/prop_attr_reflection_test.dart
index 927ebabd0214a84dd9bf13621f770ce15ef3f637..f2901c4baba88dbda2a6e929b397df9ac6f966bf 100644
--- a/pkg/polymer/test/prop_attr_reflection_test.dart
+++ b/pkg/polymer/test/prop_attr_reflection_test.dart
@@ -40,8 +40,7 @@ Future onAttributeChange(Element node) {
   return completer.future;
 }
 
-@initMethod
-main() {
+main() => initPolymer().run(() {
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);
@@ -116,4 +115,4 @@ main() {
           'reflect property based on current type');
     });
   });
-}
+});