Only preserve AC for user generators

[jridgewell]

Based on our last meeting, we'll only preserve the ambient AC state for user created generators. Spec created generators, used during iteration and async iteration, do not need to preserve the context. It would only be observable during iteration of array.values() with a getter.

Note that the if the user uses async iteration of of a spec-created generator, then their context will be preserved in their execution:

const array = [];
Object.defineProperty(array, 0, {
  get() {
    // This used to be "foo", the iterator captured ambient AC during init.
    assert.sameValue(asyncVar.get(), "bar");
  }
});

const iter = asyncVar.run("foo", () => array.values());

asyncVar.run("bar", () => {
  iter.next();
});

[andreubotella]

You forgot to check whether the mapping is empty in GeneratorResumeAbrupt.

Otherwise, LGTM.