Implement setPosition on File to seek to a byte position.

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 108 matching lines @@
   void writeString(String string);
 
   /**
    * Synchronously write a single string to the file. Returns the number
    * of characters successfully written.
    */
   // TODO(ager): writeStringSync should take an encoding.
   int writeStringSync(String string);
 
   /**
-   * Get the current position of the file. When the operation
+   * Get the current byte position in the file. When the operation
    * completes the [positionHandler] is called with the position.
    */
   void position();
 
   /**
-   * Synchronously get the current position of the file.
+   * Synchronously get the current byte position in the file.
    */
   int positionSync();
 
   /**
+   * Set the byte position in the file. When the operation completes
+   * the [setPositionHandler] is called.
+   */
+  void setPosition(int position);
+
+  /**
+   * Synchronously set the byte position in the file.
+   */
+  void setPositionSync(int position);
+
+  /**
    * 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 40 matching lines @@
 
   // Event handlers.
   void set existsHandler(void handler(bool exists));
   void set createHandler(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 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 this.message = ""]);
   String toString() => "FileIOException: $message";
   final String message;
 }