FWIW (I work on the .NET Team and I wrote the mirror we use to keep TFS and GitHub in sync). When we started the project we maintained a mirror which kept our GitHub repository and internal TFS branch in sync. After a while, we decided that trying to maintain this was more harm that it was worth (the internal TFS branch used a completely different build system and there were other interactions between code which had been open source and code that hadn't that meant our internal branch was on the floor every few days), in addition it meant that we were carrying around a bunch of effectively dead code in the source tree.

We still do mirror some code (mainly the JIT) into TFS to make it easier to share code with the Desktop in an automated fashion. However, for the rest of the code (e.g. the VM and BCL), if there are improvements we want to bring back, an engineer will just port them manually.