bool matches(item, Map matchState)

This does the matching of the actual vs expected values. item is the actual value. matchState can be supplied and may be used to add details about the mismatch that are too costly to determine in describeMismatch.

Source 

bool matches(item, Map matchState) {
  if (item is! Function && item is! Future) return false;
  if (item is Future) {
    Invoker.current.addOutstandingCallback();
    // Queue up an asynchronous expectation that validates when the future
    // completes.
    item.then((value) {
      fail("Expected future to fail, but succeeded with '$value'.");
    }, onError: (error, trace) {
      if (_matcher == null) return;

      var reason;
      if (trace != null) {
        var chain = terseChain(trace,
            verbose: Invoker.current.liveTest.test.metadata.verboseTrace);
        reason = "Actual exception trace:\n"
            "  ${chain.toString().replaceAll("\n", "\n  ")}";
      }

      // Re-run [expect] to get the proper formatting.
      expect(() => throw error, this, reason: reason);
    }).then((_) => Invoker.current.removeOutstandingCallback());
    // It hasn't failed yet.
    return true;
  }

  try {
    item();
    return false;
  } catch (e, s) {
    if (_matcher == null || _matcher.matches(e, matchState)) {
      return true;
    } else {
      addStateInfo(matchState, {'exception': e, 'stack': s});
      return false;
    }
  }
}