linter: Migrate reportLint to reportAtNode in prefer_mixin

Change-Id: Ie63886a744a1b4e6c6c9db8b6527da6c7afd2860
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/425322
Commit-Queue: Samuel Rawlins <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: srawlins

pkg/linter/lib/src/rules/prefer_mixin.dart

@@ -40,7 +40,7 @@ class _Visitor extends SimpleAstVisitor<void> {
         var element = type.element3;
         if (element is MixinElement) continue;
         if (element is ClassElement && !element.isMixinClass) {
-          rule.reportLint(mixinNode, arguments: [mixinNode.name2.lexeme]);
+          rule.reportAtNode(mixinNode, arguments: [mixinNode.name2.lexeme]);
         }
       }
     }