anchor.diff:          }
       }
   
  -    void writeDataBlock(ImprovedByteBuilder builder, String blockname, Uint8List data) {
  +    void writeDataBlock(ImprovedByteBuilder builder, String blockname, [Uint8List data = null]) {
  +        if (data == null) {
  +            data = new Uint8List(0);
  +        }
  +
           builder
               ..appendInt32(data.lengthInBytes)
               ..appendAllBytes(blockname.substring(0,4).codeUnits)
  
positive.code:         ImprovedByteBuilder builder = new ImprovedByteBuilder();
  
          this.header(builder);
          this.writeIHDR(builder);
  
          return builder.toBuffer();
      }
  
negative.code: 
      Map<String, ByteBuffer> payload = <String, ByteBuffer>{};
  
      bool saveTransparency;
  
      PayloadPng(CanvasElement this.imageSource, [bool this.saveTransparency = true]) {
  
      }
  
  