assert: handle class constructors in throws()

Currently, if a class constructor is passed as the expected value to throws(), and a match is not received, the code falls through to the validation function case, which uses fn.prototype.call(). Class constructors must be called with new, so this throws a TypeError. This commit adds a try...catch, which handles the thrown error.

I'm proposing this as an alternative to #4166. The drawback I see with #4166 is that it only handles classes which extend Error. In general, it might be a good thing to wrap user defined validation functions in a try...catch anyway. Closes #3188 (closed).