add asyncFold, asyncReduce to streams

[DartBot]

This issue was originally filed by @tatumizer

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Continuing the line started by asyncMap, asyncExpand, please add other async primitives: asyncFold, asyncReduce etc. Using asyncFold, one can implement asynchronous loops without recursion.   

[iposva-google]

Added Area-Library, Library-Async, Triaged labels.

[DartBot]

This comment was originally written by @seaneagan

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Are there any known use cases of needing to return Futures from Stream event handlers ?

Even if so, it still seems like in most cases instead of stream.asyncFold(initial, f) you could do stream.asyncMap(f1).fold(initial, f2) for some suitable f1 and f2.

I think a more common case of "asynchronous loops" is knowing the list of values up front. Quiver has doWhileAsync, reduceAsync and forEachAsync for this:

http://google.github.io/quiver-dart/#quiver/quiver-async

[DartBot]

This comment was originally written by @tatumizer

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in quiver, there's a function Future reduceAsync <http://google.github.io/quiver-dart/#quiver/quiver-async@id_reduceAsync>\( Iterable <http://google.github.io/quiver-dart/#dart-core.Iterable> iterable<http://google.github.io/quiver-dart/#quiver/quiver-async@id_reduceAsync,iterable>, initialValue<http://google.github.io/quiver-dart/#quiver/quiver-async@id_reduceAsync,initialValue>, AsyncCombiner<http://google.github.io/quiver-dart/#quiver/quiver-async.AsyncCombiner> combine<http://google.github.io/quiver-dart/#quiver/quiver-async@id_reduceAsync,combine> ) Which means you know the use case for async reduce over Iterable. Take this iterable and send it somewhere, element by element, as a stream. You get use case in question :-)

[lrhn]

Removed Type-Defect label. Added Type-Enhancement label.