Change file opening operation to set position at the end of the file when FileMode.APPEND is used.

runtime/bin/file.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.
 
 
 /**
  * FileMode describes the modes in which a file can be opened.
  */
 class FileMode {
   static final READ = const FileMode(0);
@@ skipping 88 matching lines @@
    * Files can be opened in three modes:
    *
    * FileMode.READ: open the file for reading. If the file does not
    * exist the [errorHandler] is called.
    *
    * FileMode.WRITE: open the file for both reading and writing and
    * truncate the file to length zero. If the file does not exist the
    * file is created.
    *
    * FileMode.APPEND: same as FileMode.WRITE except that the file is
-   * not truncated.
+   * not truncated and the position is set to the end of the file.
    *
    * By default mode is FileMode.READ.
    */
   RandomAccessFile openSync([FileMode mode]);
 
   /**
    * Get the canonical full path corresponding to the file name. The
    * [fullPathHandler] is called with the result when the fullPath
    * operation completes.
    */
   void fullPath();
 
   /**
    * Synchronously get the canonical full path corresponding to the file name.
    */
   String fullPathSync();
 
   /**
    * Create a new independent input stream for the file. The file
    * input stream must be closed when no longer used to free up
    * system resources.
    */
   InputStream openInputStream();
 
   /**
    * Creates a new independent output stream for the file. The file
    * output stream must be closed when no longer used to free up
    * system resources.
+   *
+   * An output stream can be opened in two modes:
+   *
+   * FileMode.WRITE: create the stream and truncate the underlying
+   * file to length zero.
+   *
+   * FileMode.APPEND: create the stream and set the position to the end of
+   * the underlying file.
+   *
+   * By default the mode is FileMode.WRITE.
    */
-  OutputStream openOutputStream();
+  OutputStream openOutputStream([FileMode mode]);
 
   /**
    * Get the name of the file.
    */
   String get name();
 
   /**
    * Sets the handler that gets called when an [exists] operation
    * completes.
    */
@@ skipping 233 matching lines @@
    */
   void set errorHandler(void handler(String error));
 }
 
 
 class FileIOException implements Exception {
   const FileIOException([String this.message = ""]);
   String toString() => "FileIOException: $message";
   final String message;
 }