Fix type warning.

runtime/bin/file_impl.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.
 
 class _FileInputStream extends _BaseDataInputStream implements InputStream {
   _FileInputStream(String name) {
     _file = new File(name);
     _data = [];
     _position = 0;
     _file.errorHandler = (String s) {
@@ skipping 77 matching lines @@
   }
 
   File _file;
   List<int> _data;
   int _position;
   bool _closed = false;
 }
 
 
 class _FileOutputStream implements OutputStream {
-  _FileOutputStream(String name, int mode) {
+  _FileOutputStream(String name, FileMode mode) {
     _pendingOperations = new List<List<int>>();
     var f = new File(name);
     f.open(mode);
     f.openHandler = (openedFile) {
       _file = openedFile;
       _setupFileHandlers();
       _processPendingOperations();
     };
     f.errorHandler = (e) {
       if (_errorHandler != null) _errorHandler();
@@ skipping 1090 matching lines @@
   Function _readByteHandler;
   Function _readListHandler;
   Function _noPendingWriteHandler;
   Function _positionHandler;
   Function _setPositionHandler;
   Function _truncateHandler;
   Function _lengthHandler;
   Function _flushHandler;
   Function _errorHandler;
 }