es6_arrow_functions: incorrect work in async functions

[unlight]

Before:

const a = {
  set: async function () {
    return await Promise.resolve(1);
  },
};

After:

const a = {
  set: () => {
  return await Promise.resolve(1);  // error   'await' expressions are only allowed within async functions and at the top levels of modules. ​typescript(1308)
},
};