A few small performance tweaks

* Use string concatenation instead of Array.push & join for accumulators:
  http://stackoverflow.com/questions/7299010/why-is-string-concatenation-faster-than-array-join

Overall parser tests might be slightly faster, but the improvement seems to be
within the margin of error.

Change-Id: I5d9e4191a79253fa808c9b0a26052470e8613a79
Merge "A few small performance tweaks"