Allow callbacks to Stream methods to return Futures ala Future methods

[DartBot]

This issue was originally filed by @seaneagan

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Currently we have Future.forEach which allows taking some async action on items serially. This is nice, but the completion values of the Futures are discarded, and it lacks the rich API of Streams.

For example, I have this async method:

Future<bool> checkCommandExists(String command) {
  String commandChecker = Platform.isWindows ? 'where' : 'hash';
  return Process.run(commandChecker, [command])
      .then((ProcessResult result) => result.exitCode == 0);
}

Then I want to be able to do:

new Stream.fromIterable(['java', 'ant', 'maven'])
    .where((command) => checkCommandExists.then((exists) => !exists))
    .first
    .then((command) => throw 'Command $command not found');

But callbacks to Stream methods such as "where" and "map" doesn't treat returning Futures specially as many of the Future methods do. At least for Streams that have a concept of order, would need to buffer the later events until the callback completes.

[sethladd]

Removed Type-Defect label.
Added Type-Enhancement, Area-Library, Library-Async, Triaged labels.

[DartBot]

This comment was originally written by @seaneagan

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asyncMap and asyncExpand now exist. This can be merged with issue #18331 which covers any further methods.

[kevmoo]

Marking as fixed. Folks can follow issue #18331 for related work

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Added Fixed label.