Changes to address IFileSystem.Watch method addition

Absorbs the new IFileSystem interface. This change is to just address the breaking change introduced in IFileSystem.
Razor has to do the necessary changes to subscribe to the Watch event for expiring the modified files.

Related IFileSystem PR : https://github.com/aspnet/FileSystem/pull/24

@pranavkm 
Must've missed this the last time around - could you also remove this method?