Fixed single quote escaping

pkg/csslib/test/declaration_test.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.
 
 library declaration_test;
 
 import 'package:unittest/unittest.dart';
 import 'testing.dart';
 
 
@@ skipping 240 matching lines @@
   content: '✔';
   line-height: 43px;
   font-size: 20px;
   color: #d9d9d9;
   text-shadow: 0 -1px 0 #bfbfbf;
 }
 ''';
 
   final String generated = r'''
 .toggle:after {
-  content: "✔";
+  content: '✔';
   line-height: 43px;
   font-size: 20px;
   color: #d9d9d9;
   text-shadow: 0 -1px 0 #bfbfbf;
 }''';
 
   var stylesheet = parseCss(input, errors: errors);
 
   expect(stylesheet != null, true);
   expect(errors.isEmpty, true, reason: errors.toString());
@@ skipping 753 matching lines @@
   test('Newer CSS', testNewerCss);
   test('Media Queries', testMediaQueries);
   test('Font-Face', testFontFace);
   test('CSS file', testCssFile);
   test('Compact Emitter', testCompactEmitter);
   test('Selector Negation', testNotSelectors);
   test('IE stuff', testIE);
   test('IE declaration syntax', testIEDeclaration);
   test('Hanging bugs', testHangs);
 }