Add support for legacy matchers from package:matcher to package:checks expectations.

• Support for both Matcher and AsyncMatcher.

You probably don't want this package if you're getting started on a new project; rather, use package:checks directly. If you're using Flutter, install package:flutter_checks instead, via flutter pub add flutter_checks. It provides a much safer, more convenient API surface out-of-the-box.

If you have an existing pure-Dart project with tests using package:matcher (exposed by package:test), then you might want this package. Consider if migrating directly to package:checks would be more beneficial, as package:checks is better tested, at least, and the cost of migration is about the same.

If you have existing custom matchers, or use matchers that aren't yet ported to package:checks, you will want this package. It can be installed with dart pub add legacy_checks.

For a full example, see example/legacy_checks_example.dart.

check(42).legacyMatcher(matcher.equals(42));

This package is maintained solely for use in package:flutter_checks. We intend on strictly adhering to SemVer, but that doesn't mean that we'll shy away from ergonomics in the name of back-compat. Please, feel free to file issues, we appreciate it! We also accept PRs, but please read the contributing guidelines first (if I've gotten around to writing them— TL;DR: Don't send anything enormous our way without prior communtation, and follow to CoC).