[ VM / Service ] Handle case where a JSON RPC notification results in…

… an error

JSON RPC notifications never receive a response by design. In the VM
service, if an error needs to be returned for a request and the request
was a notification (i.e., has no ID associated with it), null is
returned. The Dart side of the service tries to create a Response object
with the null value, resulting in an exception being thrown. This change
will swallow the errors in situations where a notification RPC is
invalid.

Change-Id: If7d889150bfec51b933474f7a66fffb25e82daed
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/109200
Reviewed-by: Siva Annamalai <asiva@google.com>

sdk/lib/vmservice/message.dart

@@ -241,7 +241,12 @@ class Message {
   }
 
   void _setResponseFromPort(dynamic response) {
-    _completer.complete(new Response.from(response));
+    if (response == null) {
+      // We should only have a null response for Notifications.
+      assert(type == MessageType.Notification);
+      return null;
+    }
+    _completer.complete(Response.from(response));
   }
 
   void setResponse(String response) {

sdk/lib/vmservice/vmservice.dart

@@ -597,7 +597,13 @@ class VMService extends MessageRouter {
   }
 
   Future<Response> routeRequest(VMService _, Message message) async {
-    return new Response.from(await _routeRequestImpl(message));
+    final response = await _routeRequestImpl(message);
+    if (response == null) {
+      // We should only have a null response for Notifications.
+      assert(message.type == MessageType.Notification);
+      return null;
+    }
+    return new Response.from(response);
   }
 
   Future _routeRequestImpl(Message message) async {