Add Isolate ping functionality.

sdk/lib/_internal/lib/isolate_helper.dart

Index: sdk/lib/_internal/lib/isolate_helper.dart
diff --git a/sdk/lib/_internal/lib/isolate_helper.dart b/sdk/lib/_internal/lib/isolate_helper.dart
index 691cfe20ccf03d34063c5f351ae489a0cd24d4b1..eefab48e333fe87970c2b04804a07d35de885965 100644
--- a/sdk/lib/_internal/lib/isolate_helper.dart
+++ b/sdk/lib/_internal/lib/isolate_helper.dart
@@ -302,6 +302,18 @@ class _IsolateContext implements IsolateContext {
     _updateGlobalState();
   }
 
+  void handlePing(SendPort responsePort, int pingType) {
+    if (pingType == Isolate.PING_EVENT) {
+      _globalState.topEventLoop.enqueue(this, () {
+        responsePort.send(null);
+      }, "ping");
+    } else {
+      // There is no difference between PING_ALIVE and PING_CONTROL
+      // since we don't handle it before the control event queue.
+      responsePort.send(null);
+    }
+  }
+
   /**
    * Run [code] in the context of the isolate represented by [this].
    */
@@ -331,6 +343,9 @@ class _IsolateContext implements IsolateContext {
       case "resume":
         removePause(message[1]);
         break;
+      case "ping":
+        handlePing(message[1], message[2]);
+        break;
       default:
         print("UNKOWN MESSAGE: $message");
     }