Type narrowing on object properties lost in async IIFE

<!-- Please try to reproduce the issue with `typescript@next`. It may have already been fixed. --> **TypeScript Version:** 3.4.0-dev.20190327 <!-- Search terms you tried before logging this (so others can find this issue more easily) --> **Search Terms:** narrowing iife control flow **Code** ```ts declare const x: string | undefined; declare const y: { z: string | undefined }; function needsString(it: string) { return it; } function a() { if (!x) { throw new Error("Missing x"); } const res1 = (() => needsString(x))(); // all good, per #8849 const res2 = (async () => needsString(x))(); // ditto if (!y.z) { throw new Error("Missing z.") } const res3 = (() => needsString(y.z))(); // still good const res4 = (async () => needsString(y.z))(); // now things blow up } ``` **Expected behavior:** The call to `needsString` in the expression for res4 succeeds, like the one in `res2` and `res3`. Even though the function is async, the code in it -- at least that's before an await -- runs immediately, I believe, and so should be subject to the narrowing (esp. given that `res2` works). **Actual behavior:** Error for `needsString(y.z)`: Argument of type 'string | undefined' is not assignable to parameter of type 'string'. **Playground Link:** https://www.typescriptlang.org/play/index.html#src=declare%20const%20x%3A%20string%20%7C%20undefined%3B%0Adeclare%20const%20y%3A%20%7B%20z%3A%20string%20%7C%20undefined%20%7D%3B%0Afunction%20needsString(it%3A%20string)%20%7B%20return%20it%3B%20%7D%0A%0Afunction%20a()%20%7B%0A%20%20if%20(!x)%20%7B%0A%20%20%20%20throw%20new%20Error(%22Missing%20x%22)%3B%0A%20%20%7D%0A%20%20const%20res1%20%3D%20(()%20%3D%3E%20needsString(x))()%3B%20%2F%2F%20all%20good%2C%20per%20%238849%0A%20%20const%20res2%20%3D%20(async%20()%20%3D%3E%20needsString(x))()%3B%20%20%2F%2F%20ditto%0A%0A%20%20if%20(!y.z)%20%7B%0A%20%20%20%20throw%20new%20Error(%22Missing%20z.%22)%0A%20%20%7D%0A%0A%20%20const%20res3%20%3D%20(()%20%3D%3E%20needsString(y.z)…