allow Matcher.matches to return Future<bool>

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<img src="https://avatars.githubusercontent.com/u/444270?v=3" align="left" width="96" height="96"hspace="10"> Issue by seaneagan
Originally opened as dart-lang/sdk#10188

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Some tests cannot be completed synchronously, in which case it would make sense to return a Future<bool> as opposed to bool from Matcher.matches.

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<img src="https://avatars.githubusercontent.com/u/2156198?v=3" align="left" width="48" height="48"hspace="10"> Comment by kasperl

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Added Area-UnitTest, Triaged labels.

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<img src="https://avatars.githubusercontent.com/u/2762632?v=3" align="left" width="48" height="48"hspace="10"> Comment by gramster

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I don't like the idea of changing matches(); there are other ways to achieve this. In particular, you can return a Future from the test function.

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<img src="https://avatars.githubusercontent.com/u/444270?v=3" align="left" width="48" height="48"hspace="10"> Comment by seaneagan

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So the goal here is to allow users to define their own reusable and
composable asynchronous matchers, so it would need to be implemented in the
Matcher itself. See for example issue dart-lang/matcher#13. Another example would be
matchers for http requests to test a REST API:

test(someRestEndPoint, allOf(hasStatus(HttpStatus.OK),
hasContentType('application/json')));

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<img src="https://avatars.githubusercontent.com/u/2762632?v=3" align="left" width="48" height="48"hspace="10"> Comment by gramster

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What I'm suggesting is you could do this as:

test('', () {
  return makeRestRequest(...).then((result) {
     expect(result, allOf(hasContentType('...'), ...));
  });
});

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<img src="https://avatars.githubusercontent.com/u/2762632?v=3" align="left" width="48" height="48"hspace="10"> Comment by gramster

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Added NotPlanned label.

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<img src="https://avatars.githubusercontent.com/u/17034?v=3" align="left" width="48" height="48"hspace="10"> Comment by kevmoo

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Added Pkg-Unittest label.

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<img src="https://avatars.githubusercontent.com/u/17034?v=3" align="left" width="48" height="48"hspace="10"> Comment by kevmoo

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Removed Area-UnitTest label.
Added Area-Pkg label.