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

pkg/polymer/test/noscript_test.dart

Index: pkg/polymer/test/noscript_test.dart
diff --git a/pkg/polymer/test/noscript_test.dart b/pkg/polymer/test/noscript_test.dart
index 58a1c0e19cb6846715d47a2124789eb7c3656be9..06cec774c48994524e85c70c42a433d69ecfb411 100644
--- a/pkg/polymer/test/noscript_test.dart
+++ b/pkg/polymer/test/noscript_test.dart
@@ -18,8 +18,7 @@ Future<List<MutationRecord>> onMutation(Node node) {
   return completer.future;
 }
 
-@initMethod
-main() {
+main() => initPolymer().run(() {
   useHtmlConfiguration();
 
   var ready = Polymer.onReady.then((_) {
@@ -36,4 +35,4 @@ main() {
     expect(querySelector('x-b').shadowRoot.nodes.first.text, 'b');
     expect(querySelector('x-d').shadowRoot.nodes.first.text, 'd');
   });
-}
+});