A pairwise matcher for Iterables.
The comparator function, taking an expected and an actual argument, and
returning whether they match, will be applied to each pair in order.
description should be a meaningful name for the comparator.
Source
Matcher pairwiseCompare(
Iterable expected, bool comparator(a, b), String description) =>
new _PairwiseCompare(expected, comparator, description);