Add DesignTime code generation support for TagHelpers.

- We now generate a subset of the TagHelper runtime code during DesignTime. This enables users to see errors in the editor
- Added tests to validate design time code generation.
- Refactored runtime code generation tests (we now use a lot of their infrastructure for the design time pieces).

#208

/cc @dougbu 
It doesn't, there's just no need for it. Following the same concept that we use for the rest of Razor where `Write(DateTIme.Now)` gets translated to `__o = DateTime.Now` in design time.