Function expectAsync(Function callback, { int count: 1, int max: 0, String id, String reason })

Indicate that callback is expected to be called count number of times (by default 1).

The test framework will wait for the callback to run the count times before it considers the current test to be complete. callback may take up to six optional or required positional arguments; named arguments are not supported.

max can be used to specify an upper bound on the number of calls; if this is exceeded the test will fail. If max is 0 (the default), the callback is expected to be called exactly count times. If max is -1, the callback is allowed to be called any number of times greater than count.

Both id and reason are optional and provide extra information about the callback when debugging. id should be the name of the callback, while reason should be the reason the callback is expected to be called.

Source 

Function expectAsync(Function callback,
        {int count: 1, int max: 0, String id, String reason}) {
  if (Invoker.current == null) {
    throw new StateError("expectAsync() may only be called within a test.");
  }

  return new _ExpectedFunction(callback, count, max, id: id, reason: reason)
      .func;
}