- Implement Isolate.ping.

tests/isolate/ping_test.dart

 // Copyright (c) 2014, 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.
 
 import "dart:isolate";
 import "dart:async";
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 
 isomain1(replyPort) {
   RawReceivePort port = new RawReceivePort();
   port.handler = (v) {
     replyPort.send(v);
     if (v == 0) port.close();
   };
   replyPort.send(port.sendPort);
 }
 
 void main(){
   asyncStart();
   var completer = new Completer();  // Completed by first reply from isolate.
   RawReceivePort reply = new RawReceivePort(completer.complete);
   Isolate.spawn(isomain1, reply.sendPort).then((Isolate isolate) {
     List result = [];
     completer.future.then((echoPort) {
       reply.handler = (v) {
+        if (result.isEmpty) {
+          // Prevent messages overtaking each other on delivery.
+          echoPort.send(0);
+        }
         result.add(v);
         if (v == 0) {
           Expect.listEquals(["alive", "control", "event"],
                             result.where((x) => x is String).toList(),
                             "control events");
           Expect.listEquals([4, 3, 2, 1, 0],
                             result.where((x) => x is int).toList(),
                             "data events");
           Expect.isTrue(result.indexOf("alive") < result.indexOf(3),
                         "alive index < 3");
           Expect.isTrue(result.indexOf("control") < result.indexOf(2),
                         "control index < 2");
           int eventIndex = result.indexOf("event");
           Expect.isTrue(eventIndex > result.indexOf(2), "event index > 2");
-          Expect.isTrue(eventIndex < result.indexOf(1), "event index < 1");
+          Expect.isTrue(eventIndex < result.indexOf(0), "event index < 0");
           reply.close();
           asyncEnd();
         }
       };
       SendPort createPingPort(message) {
         var pingPort = new RawReceivePort();
         pingPort.handler = (_) {
           result.add(message);
           pingPort.close();
         };
         return pingPort.sendPort;
       }
       echoPort.send(4);
       isolate.ping(createPingPort("alive"), Isolate.IMMEDIATE);
       echoPort.send(3);
       isolate.ping(createPingPort("control"), Isolate.BEFORE_NEXT_EVENT);
       echoPort.send(2);
       isolate.ping(createPingPort("event"), Isolate.AS_EVENT);
       echoPort.send(1);
-      echoPort.send(0);
     });
   });
 }