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 fa3765e673cdb3a57a72d0277a7f86d140da9887..8a19dc68f8ec115055d8fa9849269fa69506318b 100644
--- a/sdk/lib/_internal/lib/isolate_helper.dart
+++ b/sdk/lib/_internal/lib/isolate_helper.dart
@@ -331,6 +331,18 @@ class _IsolateContext implements IsolateContext {
     this.errorsAreFatal = errorsAreFatal;
   }
 
+  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].
    */
@@ -369,6 +381,9 @@ class _IsolateContext implements IsolateContext {
       case 'set-errors-fatal':
         setErrorsFatal(message[1], message[2]);
         break;
+      case "ping":
+        handlePing(message[1], message[2]);
+        break;
       default:
         print("UNKNOWN MESSAGE: $message");
     }