xr.combine_nested() fails when passed nested DataSets

xr.__version__ '0.13.0'

xr.combine_nested() works when passed a nested list of DataArray objects.

da1 = xr.DataArray(name="a", data=[[0]], dims=["x", "y"])
da2 = xr.DataArray(name="b", data=[[1]], dims=["x", "y"])
da3 = xr.DataArray(name="a", data=[[2]], dims=["x", "y"])
da4 = xr.DataArray(name="b", data=[[3]], dims=["x", "y"])
xr.combine_nested([[da1, da2], [da3, da4]], concat_dim=["x", "y"])

returns

<xarray.DataArray 'a' (x: 2, y: 2)>
array([[0, 1],
       [2, 3]])
Dimensions without coordinates: x, y

but fails if passed a nested list of DataSet objects.

ds1 = da1.to_dataset()
ds2 = da2.to_dataset()
ds3 = da3.to_dataset()
ds4 = da4.to_dataset()
xr.combine_nested([[ds1, ds2], [ds3, ds4]], concat_dim=["x", "y"])

returns

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-8-c0035883fc68> in <module>
      3 ds3 = da3.to_dataset()
      4 ds4 = da4.to_dataset()
----> 5 xr.combine_nested([[ds1, ds2], [ds3, ds4]], concat_dim=["x", "y"])

~/repos/contribute/xarray/xarray/core/combine.py in combine_nested(datasets, concat_dim, compat, data_vars, coords, fill_value, join)
    462         ids=False,
    463         fill_value=fill_value,
--> 464         join=join,
    465     )
    466 

~/repos/contribute/xarray/xarray/core/combine.py in _nested_combine(datasets, concat_dims, compat, data_vars, coords, ids, fill_value, join)
    305         coords=coords,
    306         fill_value=fill_value,
--> 307         join=join,
    308     )
    309     return combined

~/repos/contribute/xarray/xarray/core/combine.py in _combine_nd(combined_ids, concat_dims, data_vars, coords, compat, fill_value, join)
    196             compat=compat,
    197             fill_value=fill_value,
--> 198             join=join,
    199         )
    200     (combined_ds,) = combined_ids.values()

~/repos/contribute/xarray/xarray/core/combine.py in _combine_all_along_first_dim(combined_ids, dim, data_vars, coords, compat, fill_value, join)
    218         datasets = combined_ids.values()
    219         new_combined_ids[new_id] = _combine_1d(
--> 220             datasets, dim, compat, data_vars, coords, fill_value, join
    221         )
    222     return new_combined_ids

~/repos/contribute/xarray/xarray/core/combine.py in _combine_1d(datasets, concat_dim, compat, data_vars, coords, fill_value, join)
    246                 compat=compat,
    247                 fill_value=fill_value,
--> 248                 join=join,
    249             )
    250         except ValueError as err:

~/repos/contribute/xarray/xarray/core/concat.py in concat(objs, dim, data_vars, coords, compat, positions, fill_value, join)
    131             "objects, got %s" % type(first_obj)
    132         )
--> 133     return f(objs, dim, data_vars, coords, compat, positions, fill_value, join)
    134 
    135 

~/repos/contribute/xarray/xarray/core/concat.py in _dataset_concat(datasets, dim, data_vars, coords, compat, positions, fill_value, join)
    363     for k in datasets[0].variables:
    364         if k in concat_over:
--> 365             vars = ensure_common_dims([ds.variables[k] for ds in datasets])
    366             combined = concat_vars(vars, dim, positions)
    367             assert isinstance(combined, Variable)

~/repos/contribute/xarray/xarray/core/concat.py in <listcomp>(.0)
    363     for k in datasets[0].variables:
    364         if k in concat_over:
--> 365             vars = ensure_common_dims([ds.variables[k] for ds in datasets])
    366             combined = concat_vars(vars, dim, positions)
    367             assert isinstance(combined, Variable)

~/repos/contribute/xarray/xarray/core/utils.py in __getitem__(self, key)
    383 
    384     def __getitem__(self, key: K) -> V:
--> 385         return self.mapping[key]
    386 
    387     def __iter__(self) -> Iterator[K]:

KeyError: 'a'

Reproduce the nested DataSet example from the issue, then start in xarray/core/combine.py and xarray/core/concat.py, following the _combine_1d and _dataset_concat paths shown in the traceback. Add a regression test for the nested DataSet case and verify that combine_nested produces the same combined dimensions and values as the DataArray example.