Add a "phase: early" filter on Test262+Prepack integration

[spectranaut]

For prepack's Test262 filtering

https://github.com/facebook/prepack/blob/master/scripts/test262-runner.js#L1142

  // Ignore phase: early tests because those are errors that babel should catch
  // not issues related to Prepack
  let phase_early = testFileContents.indexOf("  phase: early");
  let end_of_comment = testFileContents.indexOf("---*/");
  if (phase_early > 0 && phase_early < end_of_comment) return false;

[leobalter]

maybe this can be simplified by filtering the negative tests,

[labond]

Per #125 we'll move this to backlog