Regression: time attributes on PeriodIndex

The following used to work with xarray 0.9.5 but doesn't anymore with 0.9.6 or master:

import xarray as xr
import pandas as pd
import numpy as np
time = pd.period_range('2000-01', '2000-12', freq='M')
da = xr.DataArray(np.arange(12), dims=['time'], coords={'time':time})
da['time.month']

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
~/.pyvirtualenvs/py3/lib/python3.5/site-packages/xarray/core/dataarray.py in _getitem_coord(self, key)
    458         try:
--> 459             var = self._coords[key]
    460         except KeyError:

KeyError: 'time.month'

During handling of the above exception, another exception occurred:

AttributeError                            Traceback (most recent call last)
<ipython-input-1-41829b924596> in <module>()
      4 time = pd.period_range('2000-01', '2000-12', freq='M')
      5 da = xr.DataArray(np.arange(12), dims=['time'], coords={'time':time})
----> 6 da['time.month']

~/.pyvirtualenvs/py3/lib/python3.5/site-packages/xarray/core/dataarray.py in __getitem__(self, key)
    467     def __getitem__(self, key):
    468         if isinstance(key, basestring):
--> 469             return self._getitem_coord(key)
    470         else:
    471             # orthogonal array indexing

~/.pyvirtualenvs/py3/lib/python3.5/site-packages/xarray/core/dataarray.py in _getitem_coord(self, key)
    461             dim_sizes = dict(zip(self.dims, self.shape))
    462             _, key, var = _get_virtual_variable(
--> 463                 self._coords, key, self._level_coords, dim_sizes)
    464 
    465         return self._replace_maybe_drop_dims(var, name=key)

~/.pyvirtualenvs/py3/lib/python3.5/site-packages/xarray/core/dataset.py in _get_virtual_variable(variables, key, level_vars, dim_sizes)
     82             data = getattr(ref_var.dt, var_name).data
     83         else:
---> 84             data = getattr(ref_var, var_name).data
     85         virtual_var = Variable(ref_var.dims, data)
     86 

AttributeError: 'IndexVariable' object has no attribute 'month'

Start with the reproducer, then read _getitem_coord in xarray/core/dataarray.py and _get_virtual_variable in xarray/core/dataset.py, where the traceback shows the failure. Verify how PeriodIndex coordinates are handled and confirm that da['time.month'] works again without breaking other virtual coordinate access.