Code Sample, a copy-pastable example if possible

```python

Create basic Dataset

import numpy as np import pandas as pd import xarray as xr

temp = 15 + 8 * np.random.randn(2, 2, 3)

precip = 10 * np.random.rand(2, 2, 3) lon = [[-99.83, -99.32], [-99.79, -99.23]] lat = [[42.25, 42.21], [42.63, 42.59]]

for real use cases, its good practice to supply array attributes such as

units, but we won't bother here for the sake of brevity

ds = xr.Dataset({'temperature': (['x', 'y', 'time'], temp), 'precipitation': (['x', 'y', 'time'], precip)}, coords={'lon': (['x', 'y'], lon), 'lat': (['x', 'y'], lat), 'time': pd.date_range('2014-09-06', periods=3), 'reference_time': pd.Timestamp('2014-09-05')})

Both of these throw attribute errors

ds.notnull()

ds.isnull()

```

Problem description

The above methods throw AttributeError: 'Variable' object has no attribute '_constructor' errors when called on Datasets. It's not clear if this operation is currently supported, but if it's not then it probably shouldn't be exposed through the API.

Expected Output

Dataset of boolean DataArrays indicating null-ness.

Output of xr.show_versions()

Show_versions() doesn't seem to exist in 0.9.6, but presumably this covers it: Numpy version: 1.13.1 Pandas version: 0.21.0 XArray version: 0.9.6