anchor.diff:    int offset;
     BitmapFileHeader(InputBuffer b) {
       if (!isValidFile(b)) {
  -      throw 'Not a bitmap file.';
  +      throw ImageException('Not a bitmap file.');
       }
       b.skip(2);
   
  
positive.code:     //   return _rgbaFrom16(input);
      // }
      else {
        throw 'Unsupported bpp ($bpp) or compression unsupported.';
      }
    }
  
  
negative.code:     };
      final compression = map[compIndex];
      if (compression == null) {
        throw ImageException(
            "Bitmap compression $compIndex is not supported yet.");
      }
      return compression;
    }
  