- Don't read .constructor when returning a Promise from an async function.
  Instead, call out to the internals of Promise.resolve directly.
  This is done by adding back in an "optimization" from an earlier form of
  the async/await code written by Caitlin Potter.
- Async functions always return a new Promise with a distinct identity,
  even if they simply return another Promise.

R=caitp@igalia.com
BUG=v8:4483

Review-Url: https://codereview.chromium.org/2219623002
Cr-Commit-Position: refs/heads/master@{#38404}