How should I test a Future in Dart?

As an alternative, here's what I've been doing. It's similar to the answers above:

test('get by keys', () {
  Future future = asyncSetup().then((_) => store.getByKeys(["hello", "dart"]));
  future.then((values) {
    expect(values, hasLength(2));
    expect(values.contains("world"), true);
    expect(values.contains("is fun"), true);
  });
  expect(future, completes);
});

I get a reference to the future, and put all my expect statements inside the then call. Then, I register a expect(future, completes) to ensure it actually completes.

Full example of how to test with the completion matcher is as follows.

import 'package:unittest/unittest.dart';

class Compute {
  Future<Map> sumIt(List<int> data) {
    Completer completer = new Completer();
    int sum = 0;
    data.forEach((i) => sum += i);
    completer.complete({"value" : sum});
    return completer.future;
  }
}

void main() {
  test("testing a future", () {
    Compute compute = new Compute();    
    Future<Map> future = compute.sumIt([1, 2, 3]);
    expect(future, completion(equals({"value" : 6})));
  });
}

The unit test runner might not complete before this code completes. So it would seem that the unit test executed correctly. With Futures that might take longer periods of time to complete the proper way is to utilize completion matcher available in unittest package.

/**
 * Matches a [Future] that completes succesfully with a value that matches
 * [matcher]. Note that this creates an asynchronous expectation. The call to
 * `expect()` that includes this will return immediately and execution will
 * continue. Later, when the future completes, the actual expectation will run.
 *
 * To test that a Future completes with an exception, you can use [throws] and
 * [throwsA].
 */
Matcher completion(matcher) => new _Completes(wrapMatcher(matcher));

One would be tempted to do the following which would be incorrect way of unit testing a returned Future in dart. WARNING: below is an incorrect way to test Futures.

import 'package:unittest/unittest.dart';

class Compute {
  Future<Map> sumIt(List<int> data) {
    Completer completer = new Completer();
    int sum = 0;
    data.forEach((i) => sum+=i);
    completer.complete({"value":sum});
    return completer.future;
  }
}

void main() {
  test("testing a future", () {
    Compute compute = new Compute();
    compute.sumIt([1, 2, 3]).then((Map m) {
      Expect.equals(true, m.containsKey("value"));
      Expect.equals(6, m["value"]);
    });
  });
}