warn if inlining the same css file more than once

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 521 matching lines @@
     'Custom element found in document body without an '
     '"unresolved" attribute on it or one of its parents. This means '
     'your app probably has a flash of unstyled content before it '
     'finishes loading.',
     'Possible flash of unstyled content',
     '''
 Custom element found in document body without an "unresolved" attribute on it or
 one of its parents. This means your app probably has a flash of unstyled content
 before it finishes loading. See <http://goo.gl/iN03Pj> for more info.
 ''');
+
+const CSS_FILE_INLINED_MULTIPLE_TIMES = const MessageTemplate(
+    const MessageId('polymer', 42),
+    'The css file %-url-% was inlined multiple times.',
+    'A css file was inlined multiple times.',
+    '''
+Css files are inlined by default, but if you import the same one in multiple
+places you probably want to override this behavior to prevent duplicate code.
+To do this, use the following pattern to update your pubspec.yaml:
+
+    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.
+'''
+);