callback =
      (left, right, unusedEq) => {
        const result = goog.testing.asserts.findDifferences(left, right);
        return result ? (path ? path + ': ' : '') + result : undefined;
      }
Executes a custom equality function

@param {!goog.testing.asserts.EqualityFunction} comparator
@param {!Object} obj1
@param {!Object} obj2
@param {string} path of the current field being checked.
@return {?goog.testing.asserts.ComparisonResult}
@private