Fix File.writeAsString to use named optional arguments.

sdk/lib/io/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.
 
 part of dart.io;
 
 /**
  * FileMode describes the modes in which a file can be opened.
  */
 class FileMode {
@@ skipping 232 matching lines @@
    *
    * Opens the file, writes the string in the given encoding, and closes the
    * file. Returns a [:Future<File>:] that completes with this [File] object
    * once the entire operation has completed.
    *
    * By default [writeAsString] creates the file for writing and truncates the
    * file if it already exists. In order to append the bytes to an existing
    * file pass [:FileMode.APPEND:] as the optional mode parameter.
    */
   Future<File> writeAsString(String contents,
-                             [Encoding encoding = Encoding.UTF_8,
-                              FileMode mode = FileMode.WRITE]);
+                             {FileMode mode: FileMode.WRITE,
+                              Encoding encoding: Encoding.UTF_8});
 
   /**
    * Synchronously write a string to a file.
    *
    * Opens the file, writes the string in the given encoding, and closes the
    * file.
    *
    * By default [writeAsStringSync] creates the file for writing and
    * truncates the file if it already exists. In order to append the bytes
    * to an existing file pass [:FileMode.APPEND:] as the optional mode
    * parameter.
    */
   void writeAsStringSync(String contents,
-                         [Encoding encoding = Encoding.UTF_8,
-                          FileMode mode = FileMode.WRITE]);
+                         {FileMode mode: FileMode.WRITE,
+                          Encoding encoding: Encoding.UTF_8});
 
   /**
    * Get the name of the file.
    */
   String get name;
 }
 
 
 /**
  * [RandomAccessFile] provides random access to the data in a
@@ skipping 166 matching lines @@
         sb.add(" ($osError)");
       }
     } else if (osError != null) {
       sb.add(": osError");
     }
     return sb.toString();
   }
   final String message;
   final OSError osError;
 }