[analysis_server] Remove non-null assertion in TypeHierarchy

Fixes #61420

Change-Id: I1d9863a1e42c7d80ab742d3cc9648b281fba56a6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/452060
Reviewed-by: Brian Wilkerson <[email protected]>
Reviewed-by: Keerti Parthasarathy <[email protected]>
Commit-Queue: Keerti Parthasarathy <[email protected]>

Author: DanTup

pkg/analysis_server/lib/src/search/type_hierarchy.dart

@@ -159,7 +159,7 @@ class TypeHierarchyComputer {
 
 class TypeHierarchyComputerHelper {
   final Element pivotElement;
-  final LibraryElement pivotLibrary;
+  final LibraryElement? pivotLibrary;
   final ElementKind pivotKind;
   final String? pivotName;
   final bool pivotFieldFinal;
@@ -192,7 +192,7 @@ class TypeHierarchyComputerHelper {
 
     return TypeHierarchyComputerHelper(
       pivotElement,
-      pivotElement.library!,
+      pivotElement.library,
       pivotElement.kind,
       pivotElement.name,
       pivotFieldFinal,
@@ -211,6 +211,12 @@ class TypeHierarchyComputerHelper {
     if (pivotName == null) {
       return null;
     }
+
+    var pivotLibrary = this.pivotLibrary;
+    if (pivotLibrary == null) {
+      return null;
+    }
+
     ExecutableElement? result;
     // try to find in the class itself
     if (pivotKind == ElementKind.METHOD) {

pkg/analysis_server/test/lsp/implementation_test.dart

@@ -237,6 +237,10 @@ abstract class MyInterface {
     await _testMarkedContent(content, otherContent: mixinsContent);
   }
 
+  Future<void> test_never_issue61420() => _testMarkedContent('''
+      Ne^ver value = throw '';
+    ''', expectResults: false);
+
   Future<void> test_nonDartFile() async {
     newFile(pubspecFilePath, simplePubspecContent);
     await initialize();

pkg/analysis_server/test/search/type_hierarchy_test.dart

@@ -1657,6 +1657,14 @@ extension type E(A it) implements A {
     expect(itemE.memberElement, isNull);
   }
 
+  Future<void> test_never_issue61420() async {
+    addTestFile('''
+Never value = throw 'error';
+''');
+    var items = await _getTypeHierarchyOrNull('Never');
+    expect(items, isNull);
+  }
+
   void _assertMember(TypeHierarchyItem item, String search) {
     expect(item.memberElement!.location!.offset, findOffset(search));
   }
@@ -1683,6 +1691,7 @@ extension type E(A it) implements A {
     await waitForTasksFinished();
     var request = _createGetTypeHierarchyRequest(search, superOnly: superOnly);
     var response = await serverChannel.simulateRequestFromClient(request);
+    expect(response.error, isNull);
     return SearchGetTypeHierarchyResult.fromResponse(
       response,
       clientUriConverter: server.uriConverter,