anchor.diff:  class MigrationResolutionHooksImpl implements MigrationResolutionHooks {
     final FixBuilder _fixBuilder;
   
  +  final Expando<List<CollectionElement>> _collectionElements = Expando();
  +
     FlowAnalysis<AstNode, Statement, Expression, PromotableElement, DartType>
         _flowAnalysis;
   
  
positive.code: 
    @override
    List<CollectionElement> getListElements(ListLiteral node) {
      return node.elements
          .map(_transformCollectionElement)
          .where((e) => e != null)
          .toList();
  
negative.code: 
    @override
    List<CollectionElement> getSetOrMapElements(SetOrMapLiteral node) {
      return _collectionElements[node] ??= node.elements
          .map(_transformCollectionElement)
          .where((e) => e != null)
          .toList();
  