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