Add Isolate ping functionality.

sdk/lib/isolate/isolate.dart

Index: sdk/lib/isolate/isolate.dart
diff --git a/sdk/lib/isolate/isolate.dart b/sdk/lib/isolate/isolate.dart
index 47b57f969acb7189ffff618a5cccbb5d27b9db0e..3ae06e9b7c75ec0b9a055d4cfbe23f785ba2d996 100644
--- a/sdk/lib/isolate/isolate.dart
+++ b/sdk/lib/isolate/isolate.dart
@@ -29,6 +29,13 @@ class IsolateSpawnException implements Exception {
 }
 
 class Isolate {
+  /** Argument to `ping`: Ask for immediate response. */
+  static const int PING_ALIVE = 0;
+  /** Argument to `ping`: Ask for response after control events. */
+  static const int PING_CONTROL = 1;
+  /** Argument to `ping`: Ask for response after normal events. */
+  static const int PING_EVENT = 2;
+
   /**
    * Control port used to send control messages to the isolate.
    *
@@ -223,6 +230,39 @@ class Isolate {
         ..[2] = errorsAreFatal;
     controlPort.send(message);
   }
+
+  /**
+   * Request that the isolate send a response on the [responsePort].
+   *
+   * WARNING: This method is experimental and not handled on every platform yet.
+   *
+   * If the isolate is alive, it will eventually send a `null` response on
+   * the response port.
+   *
+   * The [pingType] must be one of [PING_ALIVE], [PING_CONTROL] or [PING_EVENT].
+   * The response is sent at different times depending on the ping type:
+   *
+   * * `PING_ALIVE`: The the isolate responds as soon as possible.
+   *     The response should happen no later than if sent with `PING_CONTROL`.
+   *     It may be sent earlier if the system has a way to do so.
+   * * `PING_CONTROL`: The response it not sent until all previously sent
+   *     control messages from the current isolate to the receiving isolate
+   *     have been processed. This can be used to wait for
+   *     previously sent control messages.
+   * * `PING_EVENT`: The response is not sent until all prevously sent
+   *     non-control messages from the current isolate to the receiving isolate
+   *     have been processed.
+   *     The ping effectively puts the resonse into the normal event queue after
+   *     previously sent messages.
+   *     This can be used to wait for a another event to be processed.
+   */
+  void ping(SendPort responsePort, [int pingType = PING_ALIVE]) {
+    var message = new List(3)
+        ..[0] = "ping"
+        ..[1] = responsePort
+        ..[2] = pingType;
+    controlPort.send(message);
+  }
 }
 
 /**