Rework service extensions to be safe

runtime/vm/service.cc

Index: runtime/vm/service.cc
diff --git a/runtime/vm/service.cc b/runtime/vm/service.cc
index a517f21c1336952ecd5fd4c8cf688d5d8df1de74..a94fb08b0e786f329ad07c22751a658d445e25fa 100644
--- a/runtime/vm/service.cc
+++ b/runtime/vm/service.cc
@@ -905,8 +905,14 @@ bool Service::ScheduleExtensionHandler(const String& method_name,
   arguments.SetAt(2, parameter_values);
   arguments.SetAt(3, reply_port);
   arguments.SetAt(4, id);
-  return (DartEntry::InvokeFunction(schedule_extension, arguments) ==
-          Object::bool_true().raw());
+  const Object& result = Object::Handle(
+      DartEntry::InvokeFunction(schedule_extension, arguments));
+  if (result.IsNull()) {
+    return false;
+  }
+  Isolate* isolate = Isolate::Current();
+  isolate->AppendExtensionCall(Instance::Cast(result));
+  return true;
 }