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

pkg/polymer/test/instance_attrs_test.dart

Index: pkg/polymer/test/instance_attrs_test.dart
diff --git a/pkg/polymer/test/instance_attrs_test.dart b/pkg/polymer/test/instance_attrs_test.dart
index 24f94cbb05e782157bade0a3eca0e652e818927d..94e138a834dd84bc7b38804e3f2370baf4a55b97 100644
--- a/pkg/polymer/test/instance_attrs_test.dart
+++ b/pkg/polymer/test/instance_attrs_test.dart
@@ -17,8 +17,7 @@ class MyElement extends PolymerElement {
   get attributes => super.attributes;
 }
 
-@initMethod
-main() {
+main() => initPolymer().run(() {
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);
@@ -36,4 +35,4 @@ main() {
     expect(text, " foo: 123 bar: hi baz: world ",
         reason: 'text should match expected HTML template');
   });
-}
+});