Consider this:

audit = Audit.createTaskRunner();
audit.define("foo", ...);
audit.run("foo");

audit complains:

audit.js:64 Uncaught Error: Audit.run:: undefined task.

There's a logic error in run:

        for (let i = 0; arguments.length; i++) {
          let taskLabel = arguments[i];
          if (!this._tasks.hasOwnProperty(taskLabel)) {
            _throwException('Audit.run:: undefined task.');
          } else if (this._taskSequence.includes(taskLabel)) {
            _throwException('Audit.run:: duplicate task request.');
          } else {
            this._taskSequence.push[taskLabel];
          }
        }

In the first if, hasOwnProperty is true for this, but we don't do anything with it.  It should push taskLabel to taskSequence.