We currently don't produce packages for System.Composition.*. We should do so that we've a single tree for shipping fixes and features (it's currently only producing packages in our old internal Roxel tree).
It's also worth pointing out that the Roslyn workspace layer has a dependency on the Microsoft.Composition package.
Looks @mellinoe filed this a day before you :) https://github.com/dotnet/corefx/issues/6598 so closing this one.
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