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

pkg/polymer/test/unbind_test.dart

Index: pkg/polymer/test/unbind_test.dart
diff --git a/pkg/polymer/test/unbind_test.dart b/pkg/polymer/test/unbind_test.dart
index 3ab18d6f27dda4fe0a24dabc0229c53e2e313c9e..8bf725666e47d1fc86e88ed80355dd4958302ec7 100644
--- a/pkg/polymer/test/unbind_test.dart
+++ b/pkg/polymer/test/unbind_test.dart
@@ -39,14 +39,13 @@ class XTest extends PolymerElement {
   bool get isBarValid => validBar == bar;
 }
 
-@initMethod
-main() {
+main() => initPolymer().run(() {
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);
 
   test('unbind', unbindTests);
-}
+});
 
 Future testAsync(List<Function> tests, int delayMs, [List args]) {
   if (tests.length == 0) return new Future.value();