To reproduce:

1. Open the project, attached by tester (noCompositionBug.zip)
2. Open dllTest.cs script in Monodevelop
3. In Monodevelop, go to Project -> Edit References
4. Add reference to System.ComponentModel.Composition
5. In the script, add line: using System.ComponentModel.Composition
6. Go back to Unity
7. Observe the console

Expected: references added in Monodevelop are recognized when compiling code in Unity
Actual: Unity does not recognize System.ComponentMode.Composition namespace even though it is added in Monodevelop

Reproduced in 2017.1.0b1, 2017.1.0b2, 2017.1.0b3, 2017.1.0b4, 2017.1.0b5
Could not test for regression, because System.ComponentModel.Composition was introduced in .NET 4.6 and you could not use .NET 4.6 Scripting Runtime Version in 5.6 or 5.5.

Won't fix: The synchronization between the .csproj file and the Unity editor is only one-way, from Unity to the .csproj. So changes made in Visual Studio to add a project reference will not be seen by Unity. To add a reference to an assembly in the class libraries, the mcs.rsp file must be used. In general, mcs.rsp can provide the C# compiler with many different command line arguments to control the build