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Weak reference sets are similar to "populations" or "weak pairs". Like these structures, objects in a key set preserve members of an associated value set. Unlike these structures, a weak reference set is not a general associative container. Weak reference sets exist for the duration of a garbage collection. Weak reference sets are proceesed iteratively allowing values of one weak reference to be keys of a other weak reference sets. The garbage collector will process weak reference sets iteratively, revisiting all unmarked weak reference sets if any one weak reference set is marked. This process repeates until a fixed-point is reached. Review URL: https://chromiumcodereview.appspot.com//9531001 git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@4917 260f80e4-7a28-3924-810f-c04153c831b5