Fix #302: Drag and drop for linked files
master doesn't currently build.

A lot has been merged back and forth between vnext and master, and the interface `ISymbolReference` doesn't exist yet.
This commit just comments out the problem for now.
Merge pull request #473 from rojepp/fix_build

master doesn't currently build.
Merge pull request #474 from rojepp/improve_drag_drop

Fix #302: Drag and drop for linked files