[element model] migrate unnecessary_null_aware_assignments

Bug: https://github.com/dart-lang/linter/issues/5099
Change-Id: I43f854025a1bd73d5df7558e6e195ab68d229eea
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/387300
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Phil Quitslund <[email protected]>

Author: pq

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

@@ -5,7 +5,7 @@
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
-import 'package:analyzer/dart/element/element.dart';
+import 'package:analyzer/dart/element/element2.dart';
 
 import '../analyzer.dart';
 import '../extensions.dart';
@@ -38,8 +38,7 @@ class _Visitor extends SimpleAstVisitor<void> {
 
   @override
   void visitAssignmentExpression(AssignmentExpression node) {
-    if (node.readElement is PropertyAccessorElement) return;
-    if (node.writeElement is PropertyAccessorElement) return;
+    if (node.writeElement2 is SetterElement) return;
 
     if (node.operator.type == TokenType.QUESTION_QUESTION_EQ &&
         node.rightHandSide.isNullLiteral) {