Integrate has undefined name 'dim'

[max-sixty]

https://github.com/pydata/xarray/blob/master/xarray/core/dataset.py#L4085

Should that be called coord or dim? Currently there's a variable that's undefined:

            raise ValueError('Coordinate {} does not exist.'.format(dim))

I would have made a quick fix but not sure the correct name

[shoyer]

I think it should be coord. Also we should add a unit test :)

[max-sixty]

Closed by #3010