Promise implementation isn't correct

[mariusGundersen]

While trying to implement Promise.finally I have tried to understand the implementation of promises in espruino, and it does not appear to work correctly. The chaining of promises can't work right, since promises form a DAG, not a chain. I think the promise implementation could be simplified and made to work correctly if the chain code isn't used.

Here is an example that fails in today:

function delay(ms){
  return new Promise(r => setTimeout(r, ms));
}

var p = delay(1000);

p.then(() => 'a').then(x => console.log(x)); //logs a after a second
p.then(() => 'b').then(x => console.log(x)); //should log b after a second, but logs a instead

This is probably because jswrap_promise_then calls on jswrap_promise_get_chained_promise which checks if the promise has been chained already and uses the chained promise if it has. Therefore the two thens return the same promise, which they obviously shouldn't:

var p = new Promise();

var a = p.then(() => 'a');
var b = p.then(() => 'b');

console.log(a === b); // logs true

They should both return a new unresolved promise

[mariusGundersen]

This has been fixed in #2454