Interpolation using non-dimension coordinates

MCVE Code Sample

>>> da = xr.DataArray([1., 2.], coords=[('dim', [.5, 1.])])
>>> da.coords['nondim'] = ('dim', [0., 1.])
>>> da
<xarray.DataArray (dim: 2)>
array([1., 2.])
Coordinates:
  * dim      (dim) float64 0.5 1.0
    nondim   (dim) float64 0.0 1.0
>>> da.interp(dim=.75)
<xarray.DataArray ()>
array(1.5)
Coordinates:
    nondim   float64 0.5
    dim      float64 0.75
>>> da.interp(nondim=.5)
Traceback (most recent call last):

  File "<ipython-input-192-e3df34cff90f>", line 1, in <module>
    da.interp(nondim=.5)

  File "/usr/lib64/python3.6/site-packages/xarray/core/dataarray.py", line 951, in interp
    **coords_kwargs)

  File "/usr/lib64/python3.6/site-packages/xarray/core/dataset.py", line 1860, in interp
    indexers = OrderedDict(self._validate_indexers(coords))

  File "/usr/lib64/python3.6/site-packages/xarray/core/dataset.py", line 1316, in _validate_indexers
    raise ValueError("dimensions %r do not exist" % invalid)

ValueError: dimensions ['nondim'] do not exist

Expected Output

The same interpolation as for 'dim'.

Problem Description

Apparently, xarray currently cannot interpolate using non-dimension coordinates.

Output of xr.show_versions()

Reproduce the MCVE, then start at DataArray.interp and Dataset.interp in the traceback and follow _validate_indexers in xarray/core/dataset.py. Compare interpolation by the dimension coordinate with the requested non-dimension coordinate; done means da.interp(nondim=.5) produces the same interpolated result as da.interp(dim=.75).