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.
- These tests validate that our extensible directives do not have code that explodes when an incomplete directive is encountered. This is typically the case when a user is in the midst of typing a directive at design time.
- Added an extensions test and a language test.
#1271
- Added C# 7 test to validate questionable features work end-to-end.
- Had to add several explicit package references to let our VS specific packages work as expected.
#1046