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
703881154,MDExOlB1bGxSZXF1ZXN0NDg4OTA4MTI5,4432,Fix optimize for chunked DataArray,1312546,closed,0,,,8,2020-09-17T20:16:08Z,2020-09-18T13:20:45Z,2020-09-17T23:19:23Z,MEMBER,,0,pydata/xarray/pulls/4432,"Previously we generated in invalidate Dask task graph, becuase the lines
removed here dropped keys that were referenced elsewhere in the task
graph. The original implementation had a
comment indicating that this was to cull:
https://github.com/pydata/xarray/blob/502a988ad5b87b9f3aeec3033bf55c71272e1053/xarray/core/variable.py#L384

Just spot-checking things, I think we're OK here though. Something like
`dask.visualize(arr[[0]], optimize_graph=True)` indicates that we're OK.

<!-- Feel free to remove check-list items aren't relevant to your change -->

 - [x] Closes #3698
 - [x] Tests added
 - [x] Passes `isort . && black . && mypy . && flake8`
 - [x] User visible changes (including notable bug fixes) are documented in `whats-new.rst`
 - [x] New functions/methods are listed in `api.rst`
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