Improve performance of visiting large invalid compilation units

pkg/analyzer/lib/src/dart/ast/ast.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.src.dart.ast.ast;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/syntactic_entity.dart';
@@ skipping 2504 matching lines @@
   dynamic/*=E*/ accept/*<E>*/(AstVisitor/*<E>*/ visitor) =>
       visitor.visitCompilationUnit(this);
 
   @override
   void visitChildren(AstVisitor visitor) {
     _scriptTag?.accept(visitor);
     if (_directivesAreBeforeDeclarations) {
       _directives.accept(visitor);
       _declarations.accept(visitor);
     } else {
-      int length = sortedDirectivesAndDeclarations.length;
+      List<AstNode> sortedMembers = sortedDirectivesAndDeclarations;
+      int length = sortedMembers.length;
       for (int i = 0; i < length; i++) {
-        AstNode child = sortedDirectivesAndDeclarations[i];
+        AstNode child = sortedMembers[i];
         child.accept(visitor);
       }
     }
   }
 }
 
 /**
  * A node that declares one or more names within the scope of a compilation
  * unit.
  *
@@ skipping 8929 matching lines @@
 
   @override
   dynamic/*=E*/ accept/*<E>*/(AstVisitor/*<E>*/ visitor) =>
       visitor.visitYieldStatement(this);
 
   @override
   void visitChildren(AstVisitor visitor) {
     _expression?.accept(visitor);
   }
 }