Add covariant to `ThemeExtension.lerp` (#105207)

* Add covariant to `ThemeExtension.lerp`

* x

* x

examples/api/lib/material/theme/theme_extension.1.dart

@@ -26,7 +26,7 @@ class MyColors extends ThemeExtension<MyColors> {
   }
 
   @override
-  MyColors lerp(ThemeExtension<MyColors>? other, double t) {
+  MyColors lerp(MyColors? other, double t) {
     if (other is! MyColors) {
       return this;
     }

packages/flutter/lib/src/material/theme_data.dart

@@ -87,7 +87,7 @@ abstract class ThemeExtension<T extends ThemeExtension<T>> {
   /// Linearly interpolate with another [ThemeExtension] object.
   ///
   /// {@macro dart.ui.shadow.lerp}
-  ThemeExtension<T> lerp(ThemeExtension<T>? other, double t);
+  ThemeExtension<T> lerp(covariant ThemeExtension<T>? other, double t);
 }
 
 // Deriving these values is black magic. The spec claims that pressed buttons

packages/flutter/test/material/theme_data_test.dart

@@ -25,7 +25,7 @@ class MyThemeExtensionA extends ThemeExtension<MyThemeExtensionA> {
   }
 
   @override
-  MyThemeExtensionA lerp(ThemeExtension<MyThemeExtensionA>? other, double t) {
+  MyThemeExtensionA lerp(MyThemeExtensionA? other, double t) {
     if (other is! MyThemeExtensionA) {
       return this;
     }
@@ -52,7 +52,7 @@ class MyThemeExtensionB extends ThemeExtension<MyThemeExtensionB> {
   }
 
   @override
-  MyThemeExtensionB lerp(ThemeExtension<MyThemeExtensionB>? other, double t) {
+  MyThemeExtensionB lerp(MyThemeExtensionB? other, double t) {
     if (other is! MyThemeExtensionB) {
       return this;
     }