Merge pull request dotnet/coreclr#20628 from stephentoub/asyncinterfaces
authorStephen Toub <stoub@microsoft.com>
Fri, 26 Oct 2018 15:26:10 +0000 (08:26 -0700)
committerGitHub <noreply@github.com>
Fri, 26 Oct 2018 15:26:10 +0000 (08:26 -0700)
Add interfaces/helper types for async iterators

Commit migrated from https://github.com/dotnet/coreclr/commit/ab9d3795d03008e96716bd92b031c7ee57bc456a


Trivial merge