[analyzer] Add tests for null-aware elements in closing labels

Part of #56989

Change-Id: I9d402523ea065eaac4c4edbba2461a2ed23cac27
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/396600
Commit-Queue: Chloe Stefantsova <[email protected]>
Reviewed-by: Keerti Parthasarathy <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

pkg/analysis_server/test/src/computer/closing_labels_computer_test.dart

@@ -282,6 +282,42 @@ void f(HighlightRegionType type, int offset, int length) {
     _compareLabels(labels, content, expectedLabelCount: 2);
   }
 
+  Future<void> test_nullAwareElement_inList() async {
+    var content = '''
+void myMethod() {
+  return /*1*/new Wrapper(
+    Widget.createWidget(/*2*/<Widget>[
+      1,
+      ?null,
+      2
+    ]/*2:<Widget>[]*/)
+  )/*1:Wrapper*/;
+}
+''';
+
+    var labels = await _computeElements(content);
+    _compareLabels(labels, content, expectedLabelCount: 2);
+  }
+
+  Future<void> test_nullAwareElement_inList_containingList() async {
+    var content = '''
+void myMethod() {
+  return /*1*/new Wrapper(
+    Widget.createWidget(/*2*/<Widget>[
+      1,
+      ?Widget.createWidget(/*3*/<Widget>[
+        3
+      ]/*3:<Widget>[]*/),
+      2
+    ]/*2:<Widget>[]*/)
+  )/*1:Wrapper*/;
+}
+''';
+
+    var labels = await _computeElements(content);
+    _compareLabels(labels, content, expectedLabelCount: 3);
+  }
+
   Future<void> test_prefixedConstConstructor() async {
     var content = """
 import 'dart:async' as a;