Prevent incorrect namespace from breaking your REPL

Fixes #849 

Thanks to the Roslyn APIs, this requires reflection to modify an immutable, private field :)
The problem does not appear on Mono due to different namespace handling there, however once Mono error reporting is improved (there is a separate issue for that) we will have to handle that there too.
I think you can replace this `for` loop with `_invalidNamespaces.UnionWith(invalidNamespaces);`