Currently MVC is still running the IActionConstraint implementations for
features that we've already moved into the routing layer. This has a
significant perf cost associated with, and so we want to skip it because
it's redundant. However if anyone has implemented their own
`IActionConstraint`-based features, they still need to just work.
This change takes the approach of skipping the action constraint phase
at runtime unless we see something 'unknown'. This is an all or nothing
choice, and will run action constraints if **any** action constraint we
don't special case exists. This is the most compatible behavior (running
redundant constraints) when the application is using constraints that
the developer implemented.
Another approach I considered was to eliminate these constraints as part
of the process of building ADs. I don't think that's ideal because
people have written code that introspects action constraints. We should
consider something like this in 3.0.
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