Fix some warnings.

lib/isolate/frog/ports.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.
 
 /** Common functionality to all send ports. */
 class _BaseSendPort implements SendPort {
   /** Id for the destination isolate. */
   final int _isolateId;
 
-  _BaseSendPort(this._isolateId);
+  const _BaseSendPort(this._isolateId);
 
   static void checkReplyTo(SendPort replyTo) {
     if (replyTo !== null
         && replyTo is! _NativeJsSendPort
         && replyTo is! _WorkerSendPort
         && replyTo is! _BufferingSendPort) {
       throw new Exception("SendPort.send: Illegal replyTo port type");
     }
   }
 
@@ skipping 223 matching lines @@
     // map.getValues().forEach(_dispatch);
     map.getValues().forEach((e) => _dispatch(e));
   }
 
   visitBufferingSendPort(_BufferingSendPort port) {
     if (port._port == null) {
       ports.add(port._futurePort);
     }
   }
 }