then(onResolve, onReject, shouldCount) {
    if (shouldCount !== CountingThenable) {
      this.thenCallCount++;
    }
    return CountingThenable.resolve(
        this.internalPromise_.then(onResolve, onReject));
  }
@param {?(function(?): ?)=} onResolve
@param {?(function(?): ?)=} onReject
@param {*=} shouldCount
@return {?}