chore: Update min Dart constraint to 3.8 (#3676)

Update min Dart constraint to 3.8, which will enable us to use the
fancier collection literals.

This requires bumping the min Flutter version as well:

<img width="1892" height="1122" alt="image"
src="https://github.com/user-attachments/assets/7c7b07fc-4d96-4987-824d-9a7133ecfb85"
/>

examples/lib/stories/rendering/isometric_tile_map_example.dart

@@ -84,10 +84,10 @@ class Selector extends SpriteComponent {
   bool show = true;
 
   Selector(double s, Image image)
-      : super(
-          sprite: Sprite(image, srcSize: Vector2.all(32.0)),
-          size: Vector2.all(s),
-        );
+    : super(
+        sprite: Sprite(image, srcSize: Vector2.all(32.0)),
+        size: Vector2.all(s),
+      );
 
   @override
   void render(Canvas canvas) {