Remove support for String operator + from dart2js.

dart/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;
 
   const _BaseSendPort(this._isolateId);
@@ skipping 57 matching lines @@
         reply = _serializeMessage(reply);
       }
       _globalState.topEventLoop.enqueue(isolate, () {
         if (_receivePort._callback != null) {
           if (shouldSerialize) {
             msg = _deserializeMessage(msg);
             reply = _deserializeMessage(reply);
           }
           _receivePort._callback(msg, reply);
         }
-      }, 'receive ' + message);
+      }, 'receive $message');
     });
   }
 
   bool operator ==(var other) => (other is _NativeJsSendPort) &&
       (_receivePort == other._receivePort);
 
   int hashCode() => _receivePort._id;
 }
 
 /** A send port that delivers messages via worker.postMessage. */
@@ skipping 155 matching lines @@
     // map.getValues().forEach(_dispatch);
     map.getValues().forEach((e) => _dispatch(e));
   }
 
   visitBufferingSendPort(_BufferingSendPort port) {
     if (port._port == null) {
       ports.add(port._futurePort);
     }
   }
 }