map_blocks doesn't work when dask isn't installed

[crusaderky]

Iterative improvement on #3276 @dcherian

map_blocks crashes with ImportError if dask isn't installed, even if it's legal to run it on a DataArray/Dataset without any dask variables.
This forces writers of extension libraries to either not use map_blocks, add dask as a strict requirement, or write a switch in their own code.

Please change the code so that it works without dask (you'll need to write a stub of dask.is_dask_collection that always returns False) and add relevant tests to be triggered in our py36-bare-minimum CI environment.

[shoyer]

I agree, this would be a nice feature to support. I do not think it should block issuing a release.