anchor.diff:  
     @override
     String toString() {
  -    StringBuffer sb = new StringBuffer();
  +    StringBuffer sb = StringBuffer();
       sb.write('ImportSet(size: $length, ');
       for (var import in _collectImports()) {
         sb.write('${import.declaration.name} ');
  
positive.code:   void addConstant(ConstantValue constant, ImportSet importSet) {
      var item = pendingConstants[constant];
      if (item == null) {
        item = new ConstantWorkItem(constant, importSet);
        pendingConstants[constant] = item;
        queue.add(item);
      } else {
  
negative.code: 
    /// A cache of the result of calling `computeImportDeferName` on the keys of
    /// this map.
    final Map<ImportEntity, String> _importDeferName = {};
  
    /// A mapping from classes to their import set.
    Map<ClassEntity, ImportSet> _classToSet = {};
  
    /// A mapping from members to their import set.
    Map<MemberEntity, ImportSet> _memberToSet = {};
  
    /// A mapping from local functions to their import set.
    Map<Local, ImportSet> _localFunctionToSet = {};
  
    /// A mapping from constants to their import set.
    Map<ConstantValue, ImportSet> _constantToSet = {};
  
    Iterable<ImportEntity> get _allDeferredImports =>
        _deferredImportDescriptions.keys;
  
    /// Because the token-stream is forgotten later in the program, we cache a
    /// description of each deferred import.
    final Map<ImportEntity, ImportDescription> _deferredImportDescriptions = {};
  
    /// A lattice to compactly represent multiple subsets of imports.
    ImportSetLattice importSets = ImportSetLattice();
  