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

pkg/polymer/test/event_path_declarative_test.dart

Index: pkg/polymer/test/event_path_declarative_test.dart
diff --git a/pkg/polymer/test/event_path_declarative_test.dart b/pkg/polymer/test/event_path_declarative_test.dart
index 1d8c2d154e36a15bf9e4128be41128ba46bde336..9983d13a5eacf0268cf8cd6560b8350767b406d2 100644
--- a/pkg/polymer/test/event_path_declarative_test.dart
+++ b/pkg/polymer/test/event_path_declarative_test.dart
@@ -18,6 +18,8 @@ import 'package:polymer/polymer.dart';
 var _observedEvents = [];
 var _testFired;
 
+main() => initPolymer();
+
 @reflectable
 class XZug extends PolymerElement {
 
@@ -77,7 +79,7 @@ class XBar extends PolymerElement {
   }
 }
 
-@initMethod main() {
+@initMethod init() {
   useHtmlConfiguration();
   // TODO(sigmund): switch back to use @CustomTag. We seem to be running into a
   // problem where using @CustomTag doesn't guarantee that we register the tags