chai-as-promised broken by change to addMethod and addProperty

@keithamus @lucasfcosta @domenic

A PR submitted back in March (https://github.com/chaijs/chai/pull/642) seems to break chai-as-promised. The PR changed addMethod and addProperty so that it returns a new assertion with flags transferred over instead of returning this. However, it seems that chai-as-promised relies on the previous behavior so as not to lose the promise throughout the chain.

I haven’t yet dived in deeply enough to know if this is in an easy fix (for chai or for chai-as-promised). Just raising awareness.

[domenic]

OK, great! Happy to accept fixes to Chai as Promised once Chai 4.0 is released, but it’s good to know no existing users who respect the peer dependency warnings will be broken.

[meeber]

This problem can be resolved from within chai by adding chai-as-promised’s transferPromiseness logic before the return newAssertion in addMethod and addProperty:

var newAssertion = new chai.Assertion();
transferFlags(this, newAssertion);
if ("then" in this) newAssertion.then = this.then.bind(this);  // Resolves the problem
return newAssertion;

But I haven’t found a way to resolve it from within chai-as-promised.

Edit: Opened PR with possible solution