Xarray operations produce read-only array

I've turned on testing my Satpy package with unstable or pre-releases of some of our dependencies including numpy and xarray. I've found one error so far where in previous versions of xarray it was possible to assign to the numpy array taken from a DataArray.

MCVE Code Sample

import numpy as np
import dask.array as da
import xarray as xr

data = np.arange(15, 301, 15).reshape(2, 10)
data_arr = xr.DataArray(data, dims=('y', 'x'), attrs={'test': 'test'})
data_arr = data_arr.copy()
data_arr = data_arr.expand_dims('bands')
data_arr['bands'] = ['L']
n_arr = np.asarray(data_arr.data)
n_arr[n_arr == 45] = 5

Which results in:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-12-90dae37dd808> in <module>
----> 1 n_arr = np.asarray(data_arr.data); n_arr[n_arr == 45] = 5

ValueError: assignment destination is read-only

Expected Output

A writable array. No error.

Problem Description

If this is expected new behavior then so be it, but wanted to check with the xarray devs before I tried to work around it.

Output of xr.show_versions()

Start with the MCVE in the issue and trace how DataArray.data is produced after copy(), expand_dims(), and coordinate assignment. Check the relevant xarray and NumPy behavior to determine whether the read-only result is intentional; done means resolving or clearly documenting the expected writability and covering the reproduced case with a regression test.