id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
427398236,MDU6SXNzdWU0MjczOTgyMzY=,2856,Roundtripping between a dimension coordinate and scalar coordinate on a Dataset,6628425,closed,0,,,4,2019-03-31T13:42:39Z,2019-04-04T21:58:24Z,2019-04-04T21:58:24Z,MEMBER,,,,"#### Code Sample, a copy-pastable example if possible

In xarray 0.12.0 the following example produces a Dataset with no indexes:
```
In [1]: import xarray as xr

In [2]: da = xr.DataArray([1], [('x', [0])], name='a')

In [3]: da.to_dataset().isel(x=0).expand_dims('x').indexes
Out[3]:
```

#### Expected Output

In xarray 0.11.3 the roundtrip sequence above properly recovers the initial index along the `'x'` dimension:
```
In [1]: import xarray as xr

In [2]: da = xr.DataArray([1], [('x', [0])], name='a')

In [3]: da.to_dataset().isel(x=0).expand_dims('x').indexes
Out[3]: x: Int64Index([0], dtype='int64', name='x')
```

#### Output of ``xr.show_versions()``

<details><p>

```
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.7 | packaged by conda-forge | (default, Feb 28 2019, 02:16:08)
[GCC 4.2.1 Compatible Clang 4.0.1 (tags/RELEASE_401/final)]
python-bits: 64
OS: Darwin
OS-release: 18.2.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8
libhdf5: 1.10.1
libnetcdf: 4.6.1

xarray: 0.12.0
pandas: 0.24.2
numpy: 1.13.1
scipy: 0.19.1
netCDF4: 1.4.0
pydap: None
h5netcdf: 0.5.1
h5py: 2.8.0
Nio: None
zarr: None
cftime: 1.0.0
nc_time_axis: None
PseudonetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: 1.2.0
dask: 0.17.5
distributed: 1.21.8
matplotlib: 2.0.2
cartopy: None
seaborn: None
setuptools: 40.5.0
pip: 9.0.1
conda: None
pytest: 3.10.0
IPython: 6.4.0
sphinx: 1.7.4
```

</p></details>","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/2856/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,completed,13221727,issue