code: function arrayToDict(array, valueToKeyFn, opt_this) {
      opt_this = opt_this || this;
      var result = {};
      var length = array.length;
      for (var i = 0; i < length; i++) {
        var value = array[i];
        var key = valueToKeyFn.call(opt_this, value);
        result[key] = value;
      }
      return result;
    }
docs: Convert an array to a dictionary.
  
  Every element in the array is mapped in the dictionary to the key returned
  by the provided function:
  
  dictionary[valueToKeyFn(element)] = element;
  
  @param {!Array} array Arbitrary array.
  @param {function(*): string} valueToKeyFn Function mapping array elements
  to dictionary keys.
  @param {*=} opt_this Optional 'this' context for valueToKeyFn.