RFC: deopt tool

This PR is a response to https://github.com/facebook/react/issues/600. I pulled in @paulshen's React benchmarks and built a new runner for them. This runner looks at the `--trace-deopt` output of Node (while running in jsdom) and enforces that certain functions are optimized and not deopted.

Unfortunately the JIT behavior isn't deterministic (or perhaps jsdom isn't deterministic) so sometimes functions are optimized, sometimes they're not, and sometimes they deopt a lot.

This tool tests whether it is *possible* for a function to remain optimized for an extended period of time by running the tests multiple times and succeeding if the function stays optimized for at least one trial.

This PR is to assess if people think this is a good idea. If it is I can port it to node if people don't want Python in the repo.

This PR also includes a fix or two for @plievone's issues.