Potential promise pipelining with subclassing

pipelining.txt

var p0 = Promise.cast(null);
var p1 = p0.then(() => remotePromise);
var p2 = p1.invoke("foo");

line 2 calls remotePromise's .then method, like so:
  remotePromise.then(v => { set p1's [[Value]] to v }, r => { set p1's [[Reason]] to r })
it ALSO stores remotePromise in [[Following]]

Once remotePromise settles, i.e. once one of the two above callbacks gets called,
[[Following]] gets cleared.

The second half: what does p1.invoke (=== Promise.prototype.invoke) do. The answer:
- Checks if its [[Following]] is a promise;
    if so, returns [[Following]].invoke(...args),
    or possibly Promise.cast([[Following]].invoke(...args).
- Otherwise, falls back to Reflect.invoke([[Value]], ...args)