Initializing formals can't be optional parameters

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 221 matching lines @@
 interface FileInputStream extends InputStream {
   void close();
 }
 
 
 interface FileOutputStream extends OutputStream {
 }
 
 
 class FileIOException implements Exception {
-  const FileIOException([String this.message = ""]);
+  const FileIOException([String msg = ""]) : message = msg;
   String toString() => "FileIOException: $message";
   final String message;
 }