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

pkg/polymer/test/attr_deserialize_test.dart

Index: pkg/polymer/test/attr_deserialize_test.dart
diff --git a/pkg/polymer/test/attr_deserialize_test.dart b/pkg/polymer/test/attr_deserialize_test.dart
index 55060e1c113d769a2ac61224016e88693c9659e8..6274fdb3cef5fdb14922d97160afc5afd6cf10ac 100644
--- a/pkg/polymer/test/attr_deserialize_test.dart
+++ b/pkg/polymer/test/attr_deserialize_test.dart
@@ -19,8 +19,7 @@ class MyElement extends PolymerElement {
   @published Object json;
 }
 
-@initMethod
-main() {
+main() => initPolymer().run(() {
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);
@@ -47,4 +46,4 @@ main() {
         "{here: is, some: json, x: 123} ",
         reason: 'text should match expected HTML template');
   });
-}
+});