feat(delayWhen): add index to the selector function

[martinsik]

This PR adds current index to the selector function in delayWhenoperator.

source.delayWhen((value, index) => {
  ...
});

Related issue (if exists):
#2388

[coveralls]

Coverage increased (+0.0007%) to 97.689% when pulling 5d6291e on martinsik:add-index-to-delaywhen into 046b03d on ReactiveX:master.

[kwonoj]

Change looks fine, but I assume TypeScript users who used selector before encounter compilation error by selector signature changes?

[benlesh]

@kwonoj I don't think so... since arguments are optional in JavaScript, TypeScript shouldn't complain I don't think.

[benlesh]

@kwonoj confirmed... (a: A, b: B) => void matches (a: A) => void just fine in TS.

[kwonoj]

Thanks for confirmation. I'll check this in around today to tomorrow.

[benlesh]

Oops.... @kwonoj, we didn't want to merge this new feature until after we patched the existing minor.

[kwonoj]

@benlesh 😱 should this be reverted?

[benlesh]

@kwonoj yes, please. What I'd like to do is merge all of the minor changes after I merge all of the patches. So we can publish 5.2.1 and 5.3.0 at the same time.

[benlesh]

ugh.. so we're going to have to re-merge this later. :) @kwonoj can you open another PR with these changes? (no worries @martinsik, your github credit is already in the history :) )

[kwonoj]

Yup, will do.

[lock]

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