Utilities for Javascript plug

If "symbol" and "property" are percolating all the way into the eventual ES document, let's put "js-" in front of them. That way, we'll be able to do queries like `lang:python prop:item` when other languages have things called properties. (The query builder will take care of translating from `prop` to `js-property` and `py-property` and ORing them together: https://bugzilla.mozilla.org/show_bug.cgi?id=1030311.)