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

pkg/polymer/test/register_test.dart

Index: pkg/polymer/test/register_test.dart
diff --git a/pkg/polymer/test/register_test.dart b/pkg/polymer/test/register_test.dart
index c0ab7d3967e67371e4a82904e4ee56ea1379722a..d2194d57855630d4caaab580ae497a66b1e12140 100644
--- a/pkg/polymer/test/register_test.dart
+++ b/pkg/polymer/test/register_test.dart
@@ -40,8 +40,7 @@ class XButtonElement extends ButtonElement with Polymer, Observable {
 }
 
 
-@initMethod
-main() {
+main() => initPolymer().run(() {
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);
@@ -78,4 +77,4 @@ main() {
           reason: 'elements on page should be upgraded');
     });
   });
-}
+});