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

frog/file_system_memory.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('file_system_memory');
 
-#import('dart:dom');
 #import('file_system.dart');
 
 /**
  * [FileSystem] implementation a memory buffer.
  */
 class MemoryFileSystem implements FileSystem {
   StringBuffer buffer;
 
   MemoryFileSystem() : this.buffer = new StringBuffer();
 
   void writeString(String outfile, String text) {
     buffer.add(text);
   }
 
   String readAll(String filename) {
     return buffer.toString();
   }
 
   bool fileExists(String filename) {
     return true;
   }
 }