code: function addHandler (p1, p2, onFulfilled, onRejected) {
          // 2.2.1
          if (typeof onFulfilled === 'function') {
              p2._onFulfilled = onFulfilled;
          }
          if (typeof onRejected === 'function') {
              p2._onRejected = onRejected;
          }
  
          // 2.2.6
          if (p1._s === $pending) {
              p1[p1._c++] = p2;
          } else {
              scheduleHandler(p1, p2);
          }
  
          // # 2.2.7
          return p2;
      }
docs: Link the promise1 to the promise2.
  @private
  @param {Yaku} p1
  @param {Yaku} p2
  @param {Function} onFulfilled
  @param {Function} onRejected