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