Repro files: http://s000.tinyupload.com/?file_id=71495172019288651030

Compile files as test-912.cs -r:test-912-lib.dll -r:test-912-lib-missing.dll

Native csc result:

test-912-lib.dll: warning CS1684: Reference to type 'IFoo' claims it is defined in 'test-912-lib-missing.dll', but it could not be found

Managed csc result:
--no warning or error--