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

pkg/polymer/test/custom_event_test.dart

Index: pkg/polymer/test/custom_event_test.dart
diff --git a/pkg/polymer/test/custom_event_test.dart b/pkg/polymer/test/custom_event_test.dart
index 0b67e3e10eb1b1dcd4ec8d1bb43d458d5e2ab1af..04679a9426ba04b6e89b1967072a78bb826321bd 100644
--- a/pkg/polymer/test/custom_event_test.dart
+++ b/pkg/polymer/test/custom_event_test.dart
@@ -42,8 +42,7 @@ class TestCustomEvent extends PolymerElement {
   barBazHandler(e) => events.add(['barbaz', e]);
 }
 
-@initMethod
-main() {
+main() => initPolymer().run(() {
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);
@@ -67,4 +66,4 @@ main() {
     expect(events.map((e) => e[0]), ['foo', 'barbaz', 'foo']);
     expect(events.map((e) => e[1].detail), [123, 42, 777]);
   });
-}
+});