Change new List(n) to return fixed length list.

pkg/http/lib/src/multipart_request.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.
 
 library multipart_request;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:math';
 import 'dart:uri';
@@ skipping 147 matching lines @@
     if (file.filename != null) {
       header = '$header; filename="${encodeUri(file.filename)}"';
     }
     return '$header\r\n\r\n';
   }
 
   /// Returns a randomly-generated multipart boundary string of the given
   /// [length].
   String _boundaryString(int length) {
     var prefix = "dart-http-boundary-";
-    var list = new List<int>.fixedLength(length - prefix.length);
+    var list = new List<int>(length - prefix.length);
     for (var i = 0; i < list.length; i++) {
       list[i] = _BOUNDARY_CHARACTERS[
           _random.nextInt(_BOUNDARY_CHARACTERS.length)];
     }
     return "$prefix${new String.fromCharCodes(list)}";
   }
 }