anchor.diff:  }
   
   class SourceLocation {
  -  Element element;
  -  Token token;
  -  int offsetDelta;
  +  final Element element;
  +  final Token token;
  +  final int offsetDelta;
     SourceLocation(this.element, this.token, this.offsetDelta);
   }
  
positive.code:     buffer.add(other.toString());
    }
  
    CodeBuffer addCharCode(int charCode) {
      return add(new String.fromCharCodes([charCode]));
    }
  
negative.code:       : buffer = new StringBuffer(),
          sourceLocations = new List<SourceLocation>();
  
    int get length() => buffer.length;
  
    bool isEmpty() {
      return buffer.isEmpty();
  