open statements for F#  must be manually added when pasting/writing code

When writing C#, and pasting code into the editor, intellisense is able to add the required `using` statement. 

When pasting in in the same code (syntactically correct) intellisense does not allow me add the required `open` statement.

> VS bug [#598391](https://devdiv.visualstudio.com/DevDiv/_workitems/edit/598391)