Promise finally

[Raul-Sebastian Mihăilă]

@Ben Newman
There are 2 extra ticks, not just 1. The first one is caused by step 8
of 25.6.5.3.1 and the other one is caused by the fact that the
`thenFinally` callback passed in step 7 of 25.6.5.3 returns a promise. I'm
wondering if this trade-off is the right one.
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