Previously, indexing an ImplicitToExplicitIndexingAdapter object could directly
return an ExplicitlyIndexed object, which could not be indexed normally. This
resulted in broken behavior with dask's new `_meta` attribute.

This change almost but not entirely fixes xarray on dask master. There are
still errors raised inside two tests from dask's `blockwise_meta` helper
function:

>       return meta.astype(dtype)
E       AttributeError: 'ImplicitToExplicitIndexingAdapter' object has no attribute 'astype'