Now that we've shipped the NETStandard support package we will no longer build its content
from `master`.
It will be serviced from the `release/2.0.0` branch and the only netfx builds in `master` will
correspond to package content.
* The GenerateReferenceAssembly target can be run as a manual step on a
project. When run, it produces a source code file <Assembly>.cs, in the
reference assembly folder for the library. This source file can be used
to compile an equivalent reference assembly.
* Use the target to generate reference assemblies for the following
libraries:
* System.Collections.Immutable
* System.Reflection.Metadata
* System.Threading.Tasks.Dataflow
* System.Threading.Tasks.Extensions