Extract matchers from dart:unittest to separate library (dart:matchers)

[DartBot]

<img src="https://avatars.githubusercontent.com/u/480590?v=3" align="left" width="96" height="96"hspace="10"> Issue by Ladicek
Originally opened as dart-lang/sdk#3717

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Matchers are not only useful in unit testing, but also in a bunch of other contexts -- precondition checking, working with collections, specifying validations etc. Last time it was mentioned on the list is in https://groups.google.com/a/dartlang.org/group/misc/browse_thread/thread/cbdaa198dc9c8c97. Therefore, I propose to extract matchers from dart:unittest to a separate library, possibly called dart:matchers.

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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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also would be useful for a pattern matching construct:

match(x) {
  case (isNegative) ...
  case (isZero) ...
  case (lessThan(5)) ...
  default ...
}

[DartBot]

<img src="https://avatars.githubusercontent.com/u/3276024?v=3" align="left" width="48" height="48"hspace="10"> Comment by anders-sandholm

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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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Set owner to @gramster.

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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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They are now a separate library (although still in the unit test package).

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Added Fixed 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.

[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.