feat: add missing generic type to Stream.close

[Tienisto]

The returned future comes from the _ensureDoneFuture helper function which always returns Future<void>. Making the return type more specific should not be a breaking change because the changed classes are
private.

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[lrhn]

I'd love to land this, but it is a potentially breaking change, which is why we haven't just fixed it already.

Let's see if it breaks anything this time.

[mraleph]

@lrhn friendly ping.