Fix optimize for chunked DataArray

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/blame/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.

Closes pydata#3698

xarray/core/variable.py

@@ -501,9 +501,6 @@ def __dask_postpersist__(self):
 
     @staticmethod
     def _dask_finalize(results, array_func, array_args, dims, attrs, encoding):
-        if isinstance(results, dict):  # persist case
-            name = array_args[0]
-            results = {k: v for k, v in results.items() if k[0] == name}
         data = array_func(results, *array_args)
         return Variable(dims, data, attrs=attrs, encoding=encoding)
 

xarray/tests/test_dask.py

@@ -1607,3 +1607,10 @@ def test_more_transforms_pass_lazy_array_equiv(map_da, map_ds):
         assert_equal(map_da._from_temp_dataset(map_da._to_temp_dataset()), map_da)
         assert_equal(map_da.astype(map_da.dtype), map_da)
         assert_equal(map_da.transpose("y", "x", transpose_coords=False).cxy, map_da.cxy)
+
+
+def test_optimize():
+    a = dask.array.ones((10, 5), chunks=(1, 3))
+    arr = xr.DataArray(a).chunk(5)
+    (arr2,) = dask.optimize(arr)
+    arr2.compute()