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

pkg/polymer/test/attr_mustache_test.dart

Index: pkg/polymer/test/attr_mustache_test.dart
diff --git a/pkg/polymer/test/attr_mustache_test.dart b/pkg/polymer/test/attr_mustache_test.dart
index d8cb9403de038d9d9f9bdff067cc0297bae6303f..19fb79e4482afa81341acbc17004a9cf9647590d 100644
--- a/pkg/polymer/test/attr_mustache_test.dart
+++ b/pkg/polymer/test/attr_mustache_test.dart
@@ -44,8 +44,7 @@ class XTest extends PolymerElement {
   @observable var src = 'testSource';
 }
 
-@initMethod
-main() {
+main() => initPolymer().run(() {
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);
@@ -55,4 +54,4 @@ main() {
     final xtarget = xtest.shadowRoot.querySelector('#target');
     return xtarget.foundSrc;
   });
-}
+});