Create containsAllIn matcher

[alanrussian]

One of the matchers I use often in Java's truth library is "containsAllIn". I've found myself needing something like this in Dart a few times now.

A comparison of matchers in Truth vs. this package (credit @natebosch):

containsExactlyElementsIn(...) -> unorderedEquals(...)
containsExactlyElementsIn(...).inOrder() -> orderedEquals(...)
containsAll(Iterable matchers) => allOf(matchers.map(contains).toList())
containsAllOf(...).inOrder() -> containsAllInOrder(...)
containsAllIn -> Does not exist

[natebosch]

On thing that makes this tricky is that to match the semantics of other Iterable matchers in this package we'd want to support some looseness which means that a given element in the checked value could match more than one element of the expectation.

expect([2,1], containsAll([greaterThan(0), greaterThan(1)]);

My first greedy approach to solve this didn't work since the 2 matched the greaterThan(0) and failed to find a match for greaterThan(1). We might need to treat it like a bipartite matching problem.

[natebosch]

looks like unorderedMatches suffers the same problem
#73

[alanrussian]

Thanks for the quick fix, Nate! :-)