Change Directory.create to not fail on existing directory with that name.

runtime/bin/directory.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.
 
 /**
  * [Directory] objects are used for working with directories.
  */
 abstract class Directory {
   /**
    * Creates a directory object. The path is either an absolute path,
@@ skipping 20 matching lines @@
    * a [:Future<bool>:] that completes with the result.
    */
   Future<bool> exists();
 
   /**
    * Synchronously check whether a directory with this name already exists.
    */
   bool existsSync();
 
   /**
-   * Creates the directory with this name if it does not
-   * exist. Returns a [:Future<Directory>:] that completes with this
-   * directory once it has been created.
+   * Creates the directory with this name. If the directory already
+   * exists nothing is done. Returns a [:Future<Directory>:] that
+   * completes with this directory once it has been created. If the
+   * directory does not exist and cannot be created the future
+   * completes with an exception.
    */
   Future<Directory> create();
 
   /**
-   * Synchronously creates the directory with this name if it does not exist.
-   * Throws an exception if the directory already exists.
+   * Synchronously creates the directory with this name. If the
+   * directory already exists nothing is done. If the directory does
+   * not exist and cannot be created an exception is thrown.
    */
   void createSync();
 
   /**
    * Creates a temporary directory with a name based on the current
    * path.  This name and path is used as a template, and additional
    * characters are appended to it by the call to make a unique
    * directory name.  If the path is the empty string, a default
    * system temp directory and name are used for the template.
    *
@@ skipping 132 matching lines @@
       if (path != null) {
         sb.add(", path = $path");
       }
     }
     return sb.toString();
   }
   final String message;
   final String path;
   final OSError osError;
 }