analyzer: Make ResolutionVisitor._libraryElement final

Why did `https://dart.dev/tools/linter-rules/prefer_final_fields` not
catch this? 🤷

Change-Id: If3d45d8b86868062ddcdd7bae785abb93fed5f69
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/401322
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Samuel Rawlins <[email protected]>

Author: srawlins

pkg/analyzer/lib/src/dart/resolver/resolution_visitor.dart

@@ -68,7 +68,7 @@ class ElementHolder {
 /// 4. Resolve all [GenericFunctionType]s - set their types.
 /// 5. Rewrite AST where resolution provides a more accurate understanding.
 class ResolutionVisitor extends RecursiveAstVisitor<void> {
-  LibraryElementImpl _libraryElement;
+  final LibraryElementImpl _libraryElement;
   final TypeProvider _typeProvider;
   final CompilationUnitElementImpl _unitElement;
   final ErrorReporter _errorReporter;