Fix File.writeAsString to use named optional arguments.

sdk/lib/io/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.
 
 part of dart.io;
 
 class _FileInputStream extends _BaseDataInputStream implements InputStream {
   _FileInputStream(String name)
       : _data = const [],
         _position = 0,
@@ skipping 665 matching lines @@
     return completer.future;
   }
 
   void writeAsBytesSync(List<int> bytes, [FileMode mode = FileMode.WRITE]) {
     RandomAccessFile opened = openSync(mode);
     opened.writeListSync(bytes, 0, bytes.length);
     opened.closeSync();
   }
 
   Future<File> writeAsString(String contents,
-                             [FileMode mode = FileMode.WRITE,
-                              Encoding encoding = Encoding.UTF_8]) {
+                             {FileMode mode: FileMode.WRITE,
+                              Encoding encoding: Encoding.UTF_8}) {
     try {
       var data = _StringEncoders.encoder(encoding).encodeString(contents);
       return writeAsBytes(data, mode);
     } catch (e) {
       var completer = new Completer();
       new Timer(0, (t) => completer.completeException(e));
       return completer.future;
     }
   }
 
   void writeAsStringSync(String contents,
-                         [FileMode mode = FileMode.WRITE,
-                          Encoding encoding = Encoding.UTF_8]) {
+                         {FileMode mode: FileMode.WRITE,
+                          Encoding encoding: Encoding.UTF_8}) {
     var data = _StringEncoders.encoder(encoding).encodeString(contents);
     writeAsBytesSync(data, mode);
   }
 
   String get name => _name;
 
   void _ensureFileService() {
     if (_fileService == null) {
       _fileService = _FileUtils._newServicePort();
     }
@@ skipping 448 matching lines @@
           new FileIOException("File closed '$_name'"));
     });
     return completer.future;
   }
 
   final String _name;
   int _id;
 
   SendPort _fileService;
 }