Add a toString for CurveTween (#14679)

diff --git a/packages/flutter/lib/src/animation/tween.dart b/packages/flutter/lib/src/animation/tween.dart
index 7683e5d..02b86a2 100644
--- a/packages/flutter/lib/src/animation/tween.dart
+++ b/packages/flutter/lib/src/animation/tween.dart
@@ -327,4 +327,7 @@
     }
     return curve.transform(t);
   }
+
+  @override
+  String toString() => '$runtimeType(curve: $curve)';
 }