FLEX-34854 The spark Sort now extends the mx version, to prevent the need of changing both classes when a bug is fixed or a feature is implemented. NOTES: -this is a result of the mailing list discussion entitled 'Can we unify spark and mx Sort and SortField?'. -some members of mx.collections.Sort needed to be marked protected so that the sort() function could access them. In the next commit they will be reverted by implementing a way in which the sort() function doesn't need to be overridden. -mx.collections.Sort needed to have a new method, createEmptySortField(), which is now used by noFieldsCompare(), and that ensures that the differences in spark.collections.Sort.noFieldsCompare() are preserved without having to copy-paste the entire method.