Splitting interop support so `dart_support.js` is no longer required.

pkg/polymer/lib/src/build/messages.dart

 // 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.
 
 /// Contains all error and warning messages produced by polymer.
 library polymer.src.build.messages;
 
 import 'package:code_transformers/messages/messages.dart';
 import 'constants.dart';
 
@@ skipping 540 matching lines @@
 
     transformers:
     - polymer:
       inline_stylesheets:
         web/my_file.css: false
 
 If you would like to hide this warning and keep it inlined, do the same thing
 but assign the value to true.
 '''
 );
+
+const DART_SUPPORT_NO_LONGER_REQUIRED = const MessageTemplate(
+    const MessageId('polymer', 43),
+    'No need to include "dart_support.js" by hand anymore.',
+    '"dart_support.js" not necessary',
+    '''
+The script `packages/web_components/dart_support.js` is still used, but you no
+longer need to put it in your application's entrypoint.
+
+In the past this file served two purposes:
+
+  * to make dart2js work well with the platform polyfills, and
+  * to support registering Dart APIs for JavaScript custom elements.
+
+Now, the code from `dart_support.js` is split in two halves. The half for
+dart2js is now injected by the polymer transformers automatically during `pub
+build`. The `web_components` package provides an HTML file containing the other
+half.  Developers of packages that wrap JavaScript custom elements (like
+`core_elements` and `paper_elements`) will import that file directly, so
+application developers don't have to worry about it anymore.
+'''
+);