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 };
    }
@private
Helper function for markStart()

@param {Object} id  Timer id.
@param {number} time  Timer start time.