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

pkg/polymer/test/js_interop_test.dart

Index: pkg/polymer/test/js_interop_test.dart
diff --git a/pkg/polymer/test/js_interop_test.dart b/pkg/polymer/test/js_interop_test.dart
index fc6f41c89507f574fbf8604a08705ba92d4562e9..3704ccb93addc5a6d30b535d2459d6cb484bd6bf 100644
--- a/pkg/polymer/test/js_interop_test.dart
+++ b/pkg/polymer/test/js_interop_test.dart
@@ -16,8 +16,7 @@ class DartElement extends PolymerElement {
   DartElement.created() : super.created();
 }
 
-@initMethod
-main() {
+main() => initPolymer().run(() {
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);
@@ -32,7 +31,7 @@ main() {
 
   test('js-element in dart-element', () => testInterop(
       querySelector('dart-element').shadowRoot.querySelector('js-element')));
-}
+});
 
 testInterop(jsElem) {
   expect(jsElem.shadowRoot.text, 'FOOBAR');