Fixed isBoxComplete method.

src/main/java/dotsandboxes/DotsAndBoxesGrid.java

@@ -112,8 +112,8 @@ public class DotsAndBoxesGrid {
         }
 
         // A box is complete if the north and south horizontals and the east and west verticals have all been drawn.
-        // FIXME: You'll need to fix this code (after writing a test first).
-        return true;
+        boolean isBoxComplete = getHorizontal(x, y) && getVertical(x, y) && getVertical(x+1, y) && getHorizontal(x, y+1);
+        return isBoxComplete;
     }
 
     /** Tries to claim a box for a player. If the box is complete, sets the ownership and returns true. */