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

pkg/polymer/test/property_change_test.dart

Index: pkg/polymer/test/property_change_test.dart
diff --git a/pkg/polymer/test/property_change_test.dart b/pkg/polymer/test/property_change_test.dart
index 392db9259713f6f3376e6abda3287a78735e6af5..b8dfdb6f645027a5fe041baff5a80b456661a733 100644
--- a/pkg/polymer/test/property_change_test.dart
+++ b/pkg/polymer/test/property_change_test.dart
@@ -45,12 +45,11 @@ class XTest extends XBase {
   }
 }
 
-@initMethod
-main() {
+main() => initPolymer().run(() {
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);
 
   test('bar change detected', () => _bar.future);
   test('zonk change detected', () => _zonk.future);
-}
+});