Make closure-compiler-debugger faster

This used to be used only by the compiler team but is becoming increasingly useful to users wanting to report bugs or who are curious about how the compiler does ES6 transpilation, etc. It would be helpful for it to be much faster, as discussed in the #1943 discussion thread. I poked around in the appengine settings a little bit but I don't really know what I'm looking for. There is likely something we can do to decrease latency though.