code: function(moduleName, source, newCache) {
      var key = helpers.hashStrings([moduleName, source]);
      var entry = this._transpilerCache[key];
  
      if (entry) {
        return newCache[key] = entry;
      }
      try {
        return newCache[key] = {
          output: this.toFormat(
            source,
            this._generateEsperantoOptions(moduleName)
          ).code
        };
      } catch(err) {
        err.file = moduleName;
        throw err;
      }
    }
docs: Called on every file in a tree when used in per-file mode.
  
  First this checks whether the file contents have been previously
  transpiled by checking the previous cache. If the file has been transpiled,
  adds the previous transpiled code into the new cache. If it has not been transpiled
  it adds passed the source code along to a transpiler and adds the resulting value
  to the new cache.