Clean and create output directory when generating docs.

frog/file_system.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.
 
 /** Abstraction for file systems and utility functions to manipulate paths. */
 #library('file_system');
 
 /**
  * Abstraction around file system access to work in a variety of different
  * environments.
  */
 interface FileSystem {
   String readAll(String filename);
 
   void writeString(String outfile, String text);
 
   bool fileExists(String filename);
+
+  void createDirectory(String path, [bool recursive]);
+  void removeDirectory(String path, [bool recursive]);
 }
 
 /** Join [path1] to [path2]. */
 String joinPaths(String path1, String path2) {
   var pieces = path1.split('/');
   for (var piece in path2.split('/')) {
     if (piece == '..' && pieces.length > 0 && pieces.last() != '.'
       && pieces.last() != '..') {
       pieces.removeLast();
     } else if (piece != '') {
@@ skipping 18 matching lines @@
 
 /** Returns the file name without directory for the [path]. */
 String basename(String path) {
   int lastSlash = path.lastIndexOf('/', path.length);
   if (lastSlash == -1) {
     return path;
   } else {
     return path.substring(lastSlash + 1);
   }
 }