separated from bug 1365607. to make it easier to fix bug 1414177, it's better switching to async/await. I'm going to fix tests first, and then devtools implementation.
sorry, I realized that there are some ongoing project about replacing devtools implementation, so touching all of the existing tests might be overkill for the purpose of bug 1414177. I'll re-think about the plan. (might be better putting workaround there for now, and wait for the replacement)