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

[DartBot]

This issue was originally filed by [email protected]

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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.

[DartBot]

This comment was originally written 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 ...
}

[anders-sandholm]

Added Area-UnitTest, Triaged labels.

[gramster]

Set owner to @gramster.

[gramster]

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

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Added Fixed label.

[kevmoo]

Added Pkg-Unittest label.

[kevmoo]

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

[DartBot]

This issue has been moved to dart-lang/test#142.