anchor.diff:      assert(_variableIndexer != null);
       _variableIndexer.pushScope();
       var oldLabels = _labelIndexer;
  -    _labelIndexer = new LabelIndexer();
  +    _labelIndexer = null;
       var oldCases = _switchCaseIndexer;
  -    _switchCaseIndexer = new SwitchCaseIndexer();
  +    _switchCaseIndexer = null;
       // Note: FunctionNode has no tag.
       _typeParameterIndexer.enter(node.typeParameters);
       writeOffset(node.fileOffset);
  
positive.code:   int get numberOfStrings => index.length;
  
    int put(String string) {
      return index.putIfAbsent(string, () {
        entries.add(new StringTableEntry(string));
        return index.length;
      });
    }
  
    int operator [](String string) => index[string];
  
negative.code:   }
  
    visitLabeledStatement(LabeledStatement node) {
      if (_labelIndexer == null) {
        _labelIndexer = new LabelIndexer();
      }
      _labelIndexer.enter(node);
      writeByte(Tag.LabeledStatement);
      writeNode(node.body);
  