Switch to common benchmark class name in all 3 benchmarks
Re-org benchmark scripts to get common stuff in same place
Factor out common benchmark runner code
DRY up visitor benchmark as much as humanly possible

I might be going a bit overboard with this. That said, the numbers
remain the same so this does work. It might be a little too much
indirection, but it is DRY and continues to behave the same for
both Dart and JS.