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

pkg/polymer/example/canonicalization2/lib/b.dart

-// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-
-library canonicalization.b;
-
-import 'package:polymer/polymer.dart';
-import 'package:canonicalization/c.dart';
-import 'd.dart';
-
-int b = 0;
-@initMethod initB() {
-  b++;
-  c++;
-  d++;
-}