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

pkg/polymer/test/events_test.dart

Index: pkg/polymer/test/events_test.dart
diff --git a/pkg/polymer/test/events_test.dart b/pkg/polymer/test/events_test.dart
index b4d8ad7b820e227a2437eaad8d7fe51f99c9dad4..bcd6f443bf7789cdf9006a6166aa9dde0f637a15 100644
--- a/pkg/polymer/test/events_test.dart
+++ b/pkg/polymer/test/events_test.dart
@@ -44,8 +44,7 @@ class TestC extends TestBase {
   TestC.created() : super.created();
 }
 
-@initMethod
-main() {
+main() => initPolymer().run(() {
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);
@@ -84,4 +83,4 @@ main() {
     c2.click();
     expect(testC.clicks, ['local click under test-c (id c) on c-2']);
   });
-}
+});