fix duplicate script issue

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 562 matching lines @@
   * 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.
 '''
 );
+
+const SCRIPT_INCLUDED_MORE_THAN_ONCE = const MessageTemplate(
+    const MessageId('polymer', 44),
+    'The `%-url-%` script was included more than once.',
+    'Dart script file included more than once.',
+    '''
+Duplicate dart scripts often happen if you have multiple html imports that
+include the same script. The simplest workaround for this is to move your dart
+script to its own html file, and import that instead of the script (html imports
+are automatically deduped).
+
+For example:
+
+    <script type="application/dart" src="foo.dart"></script>
+
+Should turn into:
+
+    <link rel="import" href="foo.html">
+
+And `foo.html` should look like:
+
+    <script type="application/dart" src="foo.dart"></script>
+''');