LazilyOuterIndexedArray doesn't support slicing with slice objects

[mrocklin]

Code Sample, a copy-pastable example if possible

from xarray.core.indexing import LazilyOuterIndexedArray
import numpy as np
x = LazilyOuterIndexedArray(np.ones((5, 5)))
x[:3]

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-4-42bee9beb30a> in <module>
----> 1 x[:3]

~/workspace/xarray/xarray/core/indexing.py in __getitem__(self, indexer)
    518             array = LazilyVectorizedIndexedArray(self.array, self.key)
    519             return array[indexer]
--> 520         return type(self)(self.array, self._updated_key(indexer))
    521
    522     def __setitem__(self, key, value):

~/workspace/xarray/xarray/core/indexing.py in _updated_key(self, new_key)
    483
    484     def _updated_key(self, new_key):
--> 485         iter_new_key = iter(expanded_indexer(new_key.tuple, self.ndim))
    486         full_key = []
    487         for size, k in zip(self.array.shape, self.key.tuple):

AttributeError: 'slice' object has no attribute 'tuple'

Problem description

Dask array meta computations like to run x[:0, :0] on input arrays. This breaks with this class.

This is on master

[shoyer]

How does this end up getting called with dask? I don't think we pass LazilyOuterIndexedArray into dask.array.from_array() directly. For example, in xarray.Variable.chunk(), we always wrap it with ImplicitToExplicitIndexingAdapter, which should be the only object seen directly by dask:

xarray/xarray/core/variable.py

Lines 893 to 895 in 7e4bf86

	data = indexing.ImplicitToExplicitIndexingAdapter(
	data, indexing.OuterIndexer)
	data = da.from_array(data, chunks, name=name, lock=lock)

[mrocklin]

xarray/xarray/tests/test_duck_array_ops.py

Lines 291 to 297 in 7e4bf86

	@requires_cftime
	@requires_dask
	def test_cftime_datetime_mean_dask_error():
	times = cftime_range('2000', periods=4)
	da = DataArray(times, dims=['time']).chunk()
	with pytest.raises(NotImplementedError):
	da.mean()

(Pdb) pp times
CFTimeIndex([2000-01-01 00:00:00, 2000-01-02 00:00:00, 2000-01-03 00:00:00,
             2000-01-04 00:00:00],
            dtype='object')
(Pdb) pp da
<xarray.DataArray (time: 4)>
dask.array<shape=(4,), dtype=object, chunksize=(4,)>
Coordinates:
  * time     (time) object 2000-01-01 00:00:00 ... 2000-01-04 00:00:00
(Pdb) pp da.data
dask.array<xarray-<this-array>, shape=(4,), dtype=object, chunksize=(4,)>
(Pdb) pp da.data._meta
PandasIndexAdapter(array=CFTimeIndex([], dtype='object'), dtype=dtype('O'))