unittest: expectAsync should include reason

[DartBot]

<img src="https://avatars.githubusercontent.com/u/1081711?v=3" align="left" width="96" height="96"hspace="10"> Issue by jmesserly
Originally opened as dart-lang/sdk#13105

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adding "reason:" to expect is a nice way to include a human readable message to "expect". Would be nice if "expectAsync" methods also had this functionality.

[DartBot]

<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.

[DartBot]

<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.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/103167?v=3" align="left" width="48" height="48"hspace="10"> Comment by srawlins

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I've got this going at https://codereview.chromium.org/638433003

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cc @munificent.
Set owner to @srawlins.
Added Started label.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/103167?v=3" align="left" width="48" height="48"hspace="10"> Comment by srawlins

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Added Fixed label.