[analysis_server] Fix Go-to-Definition on extension overrides

Fixes https://github.com/Dart-Code/Dart-Code/issues/6043

Change-Id: I09c54fb6679c49b73f82ac017a19dd2b2ead280e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/503600
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Keerti Parthasarathy <keertip@google.com>
Commit-Queue: Keerti Parthasarathy <keertip@google.com>

pkg/analysis_server/test/lsp/definition_test.dart

@@ -659,6 +659,18 @@ class A {
     await testContents(contents);
   }
 
+  Future<void> test_extension() async {
+    var contents = '''
+var _ = StringEx^tension('').x();
+
+extension [!StringExtension!] on String {
+  void x() {}
+}
+''';
+
+    await testContents(contents);
+  }
+
   Future<void> test_field_underscore() async {
     var contents = '''
 class A {

pkg/analyzer_plugin/lib/src/utilities/navigation/navigation_dart.dart

@@ -446,6 +446,12 @@ class _DartNavigationComputerVisitor extends RecursiveAstVisitor<void> {
     super.visitExtensionDeclaration(node);
   }
 
+  @override
+  void visitExtensionOverride(ExtensionOverride node) {
+    computer._addRegionForElement(node.name, node.element);
+    super.visitExtensionOverride(node);
+  }
+
   @override
   void visitExtensionTypeDeclaration(ExtensionTypeDeclaration node) {
     computer._addRegionForFragment(