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

pkg/polymer/e2e_test/experimental_boot/test/double_init_test.dart

Index: pkg/polymer/e2e_test/experimental_boot/test/double_init_test.dart
diff --git a/pkg/polymer/e2e_test/experimental_boot/test/double_init_test.dart b/pkg/polymer/e2e_test/experimental_boot/test/double_init_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..fe6a2f2f08250bd8e2f7f6437ad41ad48d8d4426
--- /dev/null
+++ b/pkg/polymer/e2e_test/experimental_boot/test/double_init_test.dart
@@ -0,0 +1,19 @@
+// Copyright (c) 2014, 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 experimental_boot.test.double_init_test;
+
+import 'dart:async';
+import 'dart:js' as js;
+
+import 'package:polymer/polymer.dart';
+import 'package:unittest/html_config.dart';
+import 'package:unittest/unittest.dart';
+
+@initMethod
+init() {
+  useHtmlConfiguration();
+  test('can\'t call initPolymer when using polymer_experimental', () {
+    expect(() => initPolymer(), throwsA("Initialization was already done."));
+  });
+}