anchor.diff:    Map<ImportEntity, _DeferredImport> _importIndex = {};
   
     /// The canonical instance representing the empty import set.
  -  ImportSet _emptySet = new ImportSet();
  +  ImportSet _emptySet = ImportSet.empty();
   
     /// The import set representing the main output unit, which happens to be
     /// implemented as an empty set in our algorithm.
  
positive.code: 
  /// A canonical set of deferred imports.
  class ImportSet {
    /// Imports that are part of this set.
    ///
    /// Invariant: the order in which elements are added must respect the
    /// canonical order of all imports in [ImportSetLattice].
    final List<_DeferredImport> _imports;
  
    /// Links to other import sets in the lattice by adding one import.
    final Map<_DeferredImport, ImportSet> _transitions =
        <_DeferredImport, ImportSet>{};
  
    ImportSet([this._imports = const <_DeferredImport>[]]);
  
    /// The output unit corresponding to this set of imports, if any.
    OutputUnit unit;
  
    int get length => _imports.length;
  
    /// Create an import set that adds [import] to all the imports on this set.
    /// This assumes that import's canonical order comes after all imports in
    /// this current set. This should only be called from [ImportSetLattice],
    /// since it is where we preserve this invariant.
    ImportSet _add(_DeferredImport import) {
      ImportSet result = _transitions[import];
      if (result == null) {
        result = new ImportSet(new List.from(_imports)..add(import));
        result._transitions[import] = result;
        _transitions[import] = result;
      }
      return result;
    }
  
    @override
    String toString() {
      StringBuffer sb = new StringBuffer();
      sb.write('ImportSet(size: $length, ');
      for (var import in _imports) {
        sb.write('${import.declaration.name} ');
      }
      sb.write(')');
  
negative.code:     void addUnit(ImportSet importSet) {
        if (importSet.unit != null) return;
        var unit = new OutputUnit(false, '$counter',
            importSet._collectImports().map((i) => i.declaration).toSet());
        counter++;
        importSet.unit = unit;
        _allOutputUnits.add(unit);
  