expect(future, matcher) has an unhelpful stacktrace

[rrousselGit]

Consider:

test('some expectations', () {
  expect(
    Future.error(42),
    throwsA(42),
  );

  expect(
    Future.error(21),
    throwsA(42),
  );
});

When this test fails, the console will render:

-1: some expectations
  Expected: throws <42>
    Actual: <Instance of 'Future<dynamic>'>
     Which: threw <21>
            stack
  
  package:test_api  OutstandingWork.complete

The problem is, this doesn't tell us which specific expectation in our test is at fault.
In the given example, there are two expectations, both of which have the exact same stacktrace if they were to fail.

I am aware that await expectLater would fix this. But mistakes happen and we may use expect instead of expectLater when it'd help.
And the "await" may change the test's behavior, so those aren't strictly equivalent either.