See https://github.com/hax/test-promises/blob/master/promises-spec.md

As my understanding of latest es6 draft, my-promise implementation seem correct.

But maybe some order is not important. Especially the order of "a1 a2 a3 a4 a5 a6 b1 b2 b3 b4 b5 b6". It's relate to tj/co#180 (comment) . When I write my-promise, I found that it's impossible to avoid the leak completely, each nest promise need at least one pointer to preserve the order. bluebird/when have optimized to avoid the leak at all but not preserve the order. So if #179 is a case should be considered, we need relax ES6 spec to allow different order.