Error message for setting shaderWarmUp too late (#30145)

Developers may get confused by setting PaintingBinding.shaderWarmUp in
the wrong place. The added assert and error message help avoid that.

packages/flutter/lib/src/painting/binding.dart

@@ -53,7 +53,20 @@ mixin PaintingBinding on BindingBase, ServicesBinding {
   /// See also:
   ///
   ///  * [ShaderWarmUp], the interface of how this warm up works.
-  static ShaderWarmUp shaderWarmUp = const DefaultShaderWarmUp();
+  static ShaderWarmUp get shaderWarmUp => _shaderWarmUp;
+  static set shaderWarmUp(ShaderWarmUp value) {
+    assert(_instance == null,
+        'PaintingBinding.shaderWarmUp should only be set before the '
+        'PaintingBinding singleton instance is initialized.\n\n'
+        'Setting it after init would not affect how shaders are warmed up.\n\n'
+        'To fix this, try to change PaintingBinding.shaderWarmUp before:\n'
+        ' 1. runApp\n'
+        ' 2. WidgetsFlutterBinding.ensureInitialized\n'
+        ' 3. enableFlutterDriverExtension\n'
+    );
+    _shaderWarmUp = value;
+  }
+  static ShaderWarmUp _shaderWarmUp = const DefaultShaderWarmUp();
 
   /// The singleton that implements the Flutter framework's image cache.
   ///