title: Slice not working on dask chunked xarray's
body: <p>I have a large xarray which has time, y and x dimensions and is in dask chunks of time = 1, x = 2000, y = 2000. It looks something like this:</p>
  
  <pre><code>&lt;xarray.Dataset&gt;
  Dimensions:  (time: 59, x: 6951, y: 6963)
  Coordinates:
    * y        (y) float64 5.193e+06 5.193e+06 5.193e+06 ... 5.298e+06 5.298e+06
    * x        (x) float64 7.475e+05 7.476e+05 7.476e+05 ... 8.518e+05 8.518e+05
    * time     (time) datetime64[ns] 2017-11-11T03:51:53 ... 2018-02-27T03:40:41
  Data variables:
      green    (time, y, x) uint16 dask.array&lt;shape=(59, 6963, 6951), chunksize=(2, 1000, 1000)&gt;
      swir_1   (time, y, x) uint16 dask.array&lt;shape=(59, 6963, 6951), chunksize=(2, 1000, 1000)&gt;
      mask     (time, y, x) int8 dask.array&lt;shape=(59, 6963, 6951), chunksize=(2, 1000, 1000)&gt;
  Attributes:
      crs:      EPSG:32648
  </code></pre>
  
  <p>I'm trying to slice this code, so I can perform computations on a small part of it iteratively and create a new xarray at the end (due to having problems with the speed of doing .compute() on it). I've been trying variants around:</p>
  
  <pre><code>data.sel(x=slice(0,100),y=slice(0,100))                                
  </code></pre>
  
  <p>But this (and similar variants) just produce: </p>
  
  <pre><code>&lt;xarray.Dataset&gt;
  Dimensions:  (time: 59, x: 0, y: 0)
  Coordinates:
    * y        (y) float64 
    * x        (x) float64 
    * time     (time) datetime64[ns] 2017-11-11T03:51:53 ... 2018-02-27T03:40:41
  Data variables:
      green    (time, y, x) uint16 dask.array&lt;shape=(59, 0, 0), chunksize=(2, 0, 0)&gt;
      swir_1   (time, y, x) uint16 dask.array&lt;shape=(59, 0, 0), chunksize=(2, 0, 0)&gt;
      mask     (time, y, x) int8 dask.array&lt;shape=(59, 0, 0), chunksize=(2, 0, 0)&gt;
  Attributes:
      crs:      EPSG:32648
  </code></pre>
  
  <p>Is this problem caused by trying to do slicing on a chunked array? I've tried lining up the chunk size to be the same as the slice as well, but this has produced a similar problem. Ideally I just want to use the dask framework of the xarray's to do computations (such as creating indices and sums) of the data in parallel, but I haven't been able to get that to work, or at least, any computes take a long time (30+ mins).</p>