code: function MapBookKeeping(mapResult, keyPrefix, mapFunction, mapContext) {
    this.result = mapResult;
    this.keyPrefix = keyPrefix;
    this.func = mapFunction;
    this.context = mapContext;
    this.count = 0;
  }
docs: PooledClass representing the bookkeeping associated with performing a child
  mapping. Allows avoiding binding callbacks.
  
  @constructor MapBookKeeping
  @param {!*} mapResult Object containing the ordered map of results.
  @param {!function} mapFunction Function to perform mapping with.
  @param {?*} mapContext Context to perform mapping with.