When no service response is requested, send null back to the service isolate so it can cleanup

runtime/vm/service/message.dart

Index: runtime/vm/service/message.dart
diff --git a/runtime/vm/service/message.dart b/runtime/vm/service/message.dart
index f27a48bf19f65c8e70d5be0958d53e4e89c29932..c5c37866bd3d082eddfdf774cc01287b2ea857ef 100644
--- a/runtime/vm/service/message.dart
+++ b/runtime/vm/service/message.dart
@@ -94,7 +94,6 @@ class Message {
     final receivePort = new RawReceivePort();
     receivePort.handler = (value) {
       receivePort.close();
-      assert(value is String);
       _completer.complete(value);
     };
     var keys = _makeAllString(params.keys.toList(growable:false));
@@ -121,7 +120,6 @@ class Message {
     final receivePort = new RawReceivePort();
     receivePort.handler = (value) {
       receivePort.close();
-      assert(value is String);
       _completer.complete(value);
     };
     var keys = _makeAllString(params.keys.toList(growable:false));