code: function _markStart(id, time) {
          if (activeTests[id.id]) {
              console.error("Recursive tests with the same id are not supported. Timer id: " + id.id);
          }
  
          activeTests[id.id] = { startTime: time };
      }
docs: @private
  Helper function for markStart()
  
  @param {Object} id  Timer id.
  @param {number} time  Timer start time.