Fixes #1185

Turns out when resolving a promise with another promise, the other promise is unwrapped. There are three cases here:

• Resolving promise1 with resolved promise2: resolves promise1 with promise2's value (not promise 2 itself)
• Resolving promise1 with rejected promise2: rejects promise1 with promise2's rejection reason
• Resolving promise1 with pending promise2: promise1 will remain pending until promise2 is resolved or rejected
  • promise1 will resolve if promise2 resolves, or reject if promise2 rejects

All of this was verified to match JS behavior on the TypeScript playground