Tag Helpers: Modify TagHelper creation to activate ViewContext properties instead of inheriting from an interface.

[NTaylorMullen]

Right now TagHelpers can retrieve the RazorPage's ViewContext by implementing ICanHasViewContext, after talks with @yishaigalatzer we've decided that we don't want this interface to be publicly facing and that we can provide the ViewContext to the TagHelper in a better format.

New way:

class DivTagHelper : TagHelper
{
    [Activate]
    public ViewContext ViewContext { get; set; }
}

Note: Once aspnet/Razor/issues/182 is implemented we can decide if we want to make [Activate] take on the same behavior or if we want to make users be explicit on the [NotBound] attribute.

[yishaigalatzer]

go for it

[dougbu]

@NTaylorMullen and @yishaigalatzer could you please clarify this issue?
activators in the system other than DefaultControllerActivator (DefaultViewComponentActivator, RazorPageActivator and the DefaultTagHelperActivator proposed in #1259) all make use of ICanHasViewContext. this is required to handle properties that themselves require ViewContext and means ICanHasViewContext remains public-facing.
so how does fixing this issue address the first part of the excerpt below and what does "better" mean?

we've decided that we don't want this interface to be publicly facing and that we can provide the ViewContext to the TagHelper in a better format

separately tag helpers are already instantiated using ITypeActivator. so we already have a mechanism to give them any service.
note I can see some potential advantages to using [Activate] once [NotBound] exists (or the opposite -- Razor not binding properties by default). for example it allows tag helpers to usefully contain properties that implement ICanHasViewContext. but I'm not sure why we'd address that for Beta.

[NTaylorMullen]

3dff1ca