Display isolate message queue in Observatory debugger

runtime/vm/service/vmservice.dart

 // Copyright (c) 2013, 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 vmservice;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:isolate';
 import 'dart:typed_data';
@@ skipping 50 matching lines @@
       break;
       case Constants.ISOLATE_SHUTDOWN_MESSAGE_ID:
         runningIsolates.isolateShutdown(portId, sp);
       break;
     }
   }
 
   void _exit() {
     isolateLifecyclePort.close();
     scriptLoadPort.close();
-    for (var client in clients) {
+    // Create a copy of the set as a list because client.close() alters the set.
+    var clientsList = clients.toList();
+    for (var client in clientsList) {
       client.close();
     }
     // Call embedder shutdown hook after the internal shutdown.
     if (onShutdown != null) {
       onShutdown();
     }
     _onExit();
   }
 
   void messageHandler(message) {
@@ skipping 128 matching lines @@
 }
 
 void _registerIsolate(int port_id, SendPort sp, String name) {
   var service = new VMService();
   service.runningIsolates.isolateStartup(port_id, sp, name);
 }
 
 void _onStart() native "VMService_OnStart";
 
 void _onExit() native "VMService_OnExit";