ES3-0.5.4
ES5-0.5.2 | ES3-0.5.5

ES3-5 | Promise.group : if the first promise passed to a group was
already resolved, it instantly resolved the whole group.
Added a `_closed` property to `Promise.Group`, which states if the
group accepts new promises: while open ( !_closed ), any progress will
be delayed, preventing the problem.