This change removes the default usings for 'System' and
'System.Threading.Tasks' and adds them to the MVC template engine.
This is preparation for removing this feature from the razor options, I
wanted to get all of the intentional diff out of the way.
- Found that our extensible directive string parsing system wasn't consistent with the rest of the extensible directive tokens. Basically, if there were malformed string tokens we'd consume them and pass them along to extensible directive passes. This was a big no-no because it means extensible directive passes weren't able to rely on tokens being passed to them being well-formed.
- Fixed up existing extensible directive tests that relied on output of string tokens.
#1247
Ryan Nowak
N. Taylor Mullen