A working program always gives its developer a sense of joy, and rightfully so. However, this doesn't that mean you can leave the code as it is. On the contrary, carefully review your code for some time to see whether there is room for improvement or optimization. For example, are the names you used clear enough? The color of a hidden cell is now named simply COLOR_CODE in board.dart, renaming it to HIDDEN_CELL_COLOR_CODE makes its meaning explicit. The List object used in the Cells class can indicate that it is List<Cell>, by applying the fact that Dart lists are generic. The parameter of the every method in the Cell class is more precise—it is a function that accepts a cell and returns bool. Our onMouseDown event handler contains our game logic, so it is very important to tune it if possible. After some thought, we see that the code from the previous spiral can be improved; in the following line, the second condition after && is, in fact...