Expected "Resolution method is overspecified"

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Creating test with an async function which calls done synchronously does not result in the "Resolution method is overspecified" error.

Run the following test with mocha 6.2.2:

it('should throw overspecified resolution', async (done) => {
  done();
});

Expected behavior: Error: Resolution method is overspecified. Specify a callback *or* return a Promise; not both.

Actual behavior: The test passes.

Reproduces how often: Every time.