[element model] migrate unnecessary_getters_setters

Bug: https://github.com/dart-lang/linter/issues/5099
Change-Id: I7a4cb1ca0aef4bd00951e57ce3d110f08b56cebc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/388982
Auto-Submit: Phil Quitslund <[email protected]>
Reviewed-by: Samuel Rawlins <[email protected]>
Commit-Queue: Samuel Rawlins <[email protected]>

Author: pq

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

@@ -73,8 +73,8 @@ class _Visitor extends SimpleAstVisitor<void> {
 
   void _visitGetterSetter(MethodDeclaration getter, MethodDeclaration? setter) {
     if (setter == null) return;
-    var getterElement = getter.declaredElement;
-    var setterElement = setter.declaredElement;
+    var getterElement = getter.declaredFragment?.element;
+    var setterElement = setter.declaredFragment?.element;
     if (getterElement == null || setterElement == null) return;
     if (isSimpleSetter(setter) &&
         isSimpleGetter(getter) &&