issue_comments: 500264231

In my own tests I've been using the following custom scheduler with dask.config.set(scheduler=CustomScheduler()) to point out what code is computing the array when I don't want it to:

``` class CustomScheduler(object): """Custom dask scheduler that raises an exception if dask is computed too many times."""

def __init__(self, max_computes=1):
    """Set starting and maximum compute counts."""
    self.max_computes = max_computes
    self.total_computes = 0

def __call__(self, dsk, keys, **kwargs):
    """Compute dask task and keep track of number of times we do so."""
    import dask
    self.total_computes += 1
    if self.total_computes > self.max_computes:
        raise RuntimeError("Too many dask computations were scheduled: {}".format(self.total_computes))
    return dask.get(dsk, keys, **kwargs)

```

Does something like this exist in the xarray tests? If not, I could add it then add a dask test to the DataArray tests.