code: function () {
          function checkUpdate() {
              var current = module.exports.current();
              if (current !== last) {
                  last = current;
                  events.trigger('update', current);
              }
          }
  
          // Do not run unless breakpoints are available.
          if (!running && breakpoints.length) {
              running = true;
              global.addEventListener('resize', function () {
                  if (!working) {
                      working = true;
                      global.requestAnimationFrame(function () {
                          checkUpdate();
                          working = false;
                      });
                  }
              });
          }
          checkUpdate();
      }
docs: ## watcher.run(throttle)
  Uses RAF to monitor breakpoint changes.