Builds a textual description of a specific mismatch.
item is the value that was tested by matches;
the Map that was passed to and supplemented by matches
with additional information about the mismatch, and
is the Description that is being built to describe the mismatch.
A few matchers make use of the
verbose flag to provide detailed
information that is not typically included but can be of help in
diagnosing failures, such as stack traces.
@override Description describeMismatch( item, Description description, Map matchState, bool verbose) => StringDescription(matchState[this] as String);