fix dart analysis error (#20698)

packages/flutter_test/lib/src/matchers.dart

@@ -1188,7 +1188,7 @@ class _MatchAnythingExceptClip extends _FailWithDescriptionMatcher {
 
   @override
   Description describe(Description description) {
-    description.add('does not have a clip as an immediate child');
+    return description.add('does not have a clip as an immediate child');
   }
 }
 
@@ -1730,4 +1730,4 @@ class _DoesNotMatchAccessibilityGuideline extends AsyncMatcher {
       return 'Failed';
     return null;
   }
-}
\ No newline at end of file
+}