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

pkg/polymer/lib/polymer.dart

 // Copyright (c) 2012, 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.
 
 /// Custom HTML tags, data binding, and templates for building
 /// structured, encapsulated, client-side web apps.
 ///
 /// Polymer.dart, the next evolution of Web UI,
 /// is an in-progress Dart port of the
 /// [Polymer project](http://www.polymer-project.org/).
@@ skipping 34 matching lines @@
 
 import 'dart:async';
 import 'dart:collection' show HashMap, HashSet;
 import 'dart:html';
 import 'dart:js' as js show context;
 import 'dart:js' hide context;
 
 // *** Important Note ***
 // This import is automatically replaced when calling pub build by the
 // mirrors_remover transformer. The transformer will remove any dependencies on
-// dart:mirrors in deployed polymer apps. This should be updated in sync with
-// changed in lib/src/build/mirrors_remover.dart.
+// dart:mirrors in deployed polymer apps. This and the import to
+// mirror_loader.dart below should be updated in sync with changed in
+// lib/src/build/mirrors_remover.dart.
 //
 // Technically this annotation is not needed now that we have codegen for
 // expressions, but our test bots don't run pub-build yet. Until then, tests
 // might (transitively) have an import to smoke.mirrors, even though the code is
 // completely dead. This @MirrorsUsed annotation helps reduce the load on our
 // bots.
 @MirrorsUsed(metaTargets:
     const [Reflectable, ObservableProperty, PublishedProperty, CustomTag,
         ObserveProperty],
     targets: const [PublishedProperty, ObserveProperty],
     override: const ['smoke.mirrors'])
 import 'dart:mirrors' show MirrorsUsed;    // ** see important note above
 
 import 'package:logging/logging.dart' show Logger, Level;
 import 'package:observe/observe.dart';
+import 'package:observe/src/dirty_check.dart' show dirtyCheckZone;
 import 'package:polymer_expressions/polymer_expressions.dart'
     show PolymerExpressions;
 import 'package:smoke/smoke.dart' as smoke;
 import 'package:template_binding/template_binding.dart';
 
 import 'deserialize.dart' as deserialize;
+import 'src/mirror_loader.dart' as loader; // ** see important note above
 
 export 'package:observe/observe.dart';
 export 'package:observe/html.dart';
 
 part 'src/declaration.dart';
 part 'src/instance.dart';
 part 'src/job.dart';
 part 'src/loader.dart';