Don't depend on dart:dom in file_system_memory.dart.

utils/css/css.dart

 // Copyright (c) 2011, 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('css');
 
+#import('dart:dom');
 #import('../../frog/lang.dart', prefix:'lang');
 #import('../../frog/file_system_memory.dart');
 
 #source('tokenkind.dart');
 #source('tokenizer.dart');
 #source('tree.dart');
 #source('cssselectorexception.dart');
 #source('cssworld.dart');
 #source('parser.dart');
 
@@ skipping 36 matching lines @@
       output = prettyTree;
     }
   } catch (var e) {
     String error = e.toString();
     output = "$error\n$prettyTree";
     throw e;
   }
 
   return output;
 }