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

pkg/polymer/test/property_observe_test.dart

Index: pkg/polymer/test/property_observe_test.dart
diff --git a/pkg/polymer/test/property_observe_test.dart b/pkg/polymer/test/property_observe_test.dart
index f820e4debfc87f526362120577a7bd88c779a48f..b79fa341981afb0ce94aecc6c38ceec4819463c2 100644
--- a/pkg/polymer/test/property_observe_test.dart
+++ b/pkg/polymer/test/property_observe_test.dart
@@ -81,11 +81,10 @@ class XTest2 extends XTest {
   }
 }
 
-@initMethod
-main() {
+main() => initPolymer().run(() {
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);
 
   test('changes detected', () => _done.future);
-}
+});