Clean and create output directory when generating docs.

frog/file_system_vm.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_vm');
 #import('file_system.dart');
 
 /** File system implementation using the vm api's. */
 class VMFileSystem implements FileSystem {
   void writeString(String outfile, String text) {
     var f = new File(outfile);
     var stream = f.openOutputStream();
     stream.write(text.charCodes());
     stream.close();
   }
 
   String readAll(String filename) {
     var file = new File(filename);
     file.openSync();
     var length = file.lengthSync();
     var buffer = new List<int>(length);
     var bytes = file.readListSync(buffer, 0, length);
     file.closeSync();
     return new String.fromCharCodes(buffer);
   }
 
   bool fileExists(String filename) {
     return new File(filename).existsSync();
   }
+
+  void createDirectory(String path, [bool recursive = false]) {
+    throw 'createDirectory() is not implemented by VMFileSystem yet.';

[Jacob, 2011/11/16 19:59:30]

same comment as other file 

[Bob Nystrom, 2011/11/17 20:44:21]

Done.

+  }
+
+  void removeDirectory(String path, [bool recursive = false]) {
+    throw 'removeDirectory() is not implemented by VMFileSystem yet.';
+  }
 }