Add truncate operation to File API.

runtime/bin/file.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.
 
 interface File factory _File {
   /**
    * Create a File object.
    */
   File(String name);
 
@@ skipping 142 matching lines @@
    * the [setPositionHandler] is called.
    */
   void setPosition(int position);
 
   /**
    * Synchronously set the byte position in the file.
    */
   void setPositionSync(int position);
 
   /**
+   * Truncate (or extend) the file to [length] bytes. When the
+   * operation completes successfully the [truncateHandler] is called.
+   */
+  void truncate(int length);
+
+  /**
+   * Synchronously truncate (or extend) the file to [length] bytes.
+   */
+  void truncateSync(int length);
+
+  /**
    * Get the length of the file. When the operation completes the
    * [lengthHandler] is called with the length.
    */
   void length();
 
   /**
    * Get the length of the file. When the operation completes the
    * [lengthHandler] is called with the length.
    */
   int lengthSync();
@@ skipping 42 matching lines @@
   void set existsHandler(void handler(bool exists));
   void set createHandler(void handler());
   void set deleteHandler(void handler());
   void set openHandler(void handler());
   void set closeHandler(void handler());
   void set readByteHandler(void handler(int byte));
   void set readListHandler(void handler(int read));
   void set noPendingWriteHandler(void handler());
   void set positionHandler(void handler(int position));
   void set setPositionHandler(void handler());
+  void set truncateHandler(void handler());
   void set lengthHandler(void handler(int length));
   void set flushHandler(void handler());
   void set fullPathHandler(void handler(String path));
   void set errorHandler(void handler(String error));
 }
 
 
 interface FileInputStream extends InputStream {
   void close();
 }
 
 
 interface FileOutputStream extends OutputStream {
 }
 
 
 class FileIOException implements Exception {
   const FileIOException([String msg = ""]) : message = msg;
   String toString() => "FileIOException: $message";
   final String message;
 }