Remove support for String operator + from dart2js.

dart/samples/proxy/proxy.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.
 
 #library("proxy");
 #import("dart:isolate");
 
 /**
  * Base class for all RpcProxy's
  *
@@ skipping 204 matching lines @@
   static final String prefix = "RpcException:";
 
   final String message;
   const RpcException(String this.message);
 
   String toString() {
     return message;
   }
 
   static String format(Object e) {
-    return prefix + e.toString();
+    return "$prefix$e";
   }
 
   static RpcException parse(Object object) {
     if (object === null || !(object is String)) {
       return null;
     }
     String s = object;
     if (!s.startsWith(prefix)) {
       return null;
     }
     return new RpcException(s.substring(prefix.length, s.length));
   }
 }