Since 6101eb1 landed.

AssertionError can take message, expected and actual options, if message is not passed, it gets generated from expected and actual.

Within test runner output, this leads to some duplication in the output:

     this.assert.strictEqual('1', 1);
       "1" === 1
       expected: 1
            got: '1'

Ideally I'd like to only display the expected and actual, unless a custom message has been passed - but as it stands I can't see any way to tell the difference.