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

pkg/polymer/test/entered_view_test.dart

Index: pkg/polymer/test/entered_view_test.dart
diff --git a/pkg/polymer/test/entered_view_test.dart b/pkg/polymer/test/entered_view_test.dart
index f1859ada9d1f51648e352b0544a282789ce040fd..648a487e3155cc36b5ea141a81ea69aaee2879ab 100644
--- a/pkg/polymer/test/entered_view_test.dart
+++ b/pkg/polymer/test/entered_view_test.dart
@@ -29,8 +29,7 @@ class XInner extends PolymerElement {
   }
 }
 
-@initMethod
-main() {
+main() => initPolymer().run(() {
   useHtmlConfiguration();
   Polymer.register('x-inner', XInner);
   Polymer.register('x-outer', XOuter);
@@ -49,4 +48,4 @@ main() {
       });
     });
   });
-}
+});