code: function(aKey, aValue) {
        return isReady.then(() => {
          internalState[aKey] = aValue;
          processingWrite = true;
          return writeFile(aFilepath, JSON.stringify(internalState)).then(() => {
            return aValue;
          });
        });
      }
docs: TO-DO TO-DO: Currently when we write the file the watcher fires up and we read it again. That's suboptimal. Fix it.