fix typo in error mesage polymer#42

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 533 matching lines @@
     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
+        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.
 '''
 );