UglifyJS requires Node.js to run, but it's a lot faster than Google's
compiler and produces smaller gzipped sizes. Some of the non-gzipped
sizes are a bit larger than Google's but I think the gzipped size is
more important. Faster runtime is also good when we start testing the
minified versions too.
The renaming of attributes is totally not worth the hassle of maintaining an
externs file (or using the awkward `foo["bar"]` syntax). The file size
reduction from the advanced optimizations was negligible, besides!