Issue 27539. Check that the source is not modified between constructor and correction computing.

pkg/analysis_server/lib/src/services/correction/assist_internal.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 services.src.correction.assist;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analysis_server/plugin/edit/assist/assist_core.dart';
@@ skipping 69 matching lines @@
     selectionLength = dartContext.selectionLength;
     selectionEnd = selectionOffset + selectionLength;
   }
 
   /**
    * Returns the EOL to use for this [CompilationUnit].
    */
   String get eol => utils.endOfLine;
 
   Future<List<Assist>> compute() async {
+    // If the source was changed between the constructor and running
+    // this asynchronous method, it is not safe to use the unit.
+    if (analysisContext.getModificationStamp(source) != fileStamp) {
+      return const <Assist>[];
+    }
+
     try {
       utils = new CorrectionUtils(unit);
     } catch (e) {
       throw new CancelCorrectionException(exception: e);
     }
 
     node = new NodeLocator(selectionOffset, selectionEnd).searchWithin(unit);
     if (node == null) {
       return assists;
     }
@@ skipping 2157 matching lines @@
 class _SimpleIdentifierRecursiveAstVisitor extends RecursiveAstVisitor {
   final _SimpleIdentifierVisitor visitor;
 
   _SimpleIdentifierRecursiveAstVisitor(this.visitor);
 
   @override
   visitSimpleIdentifier(SimpleIdentifier node) {
     visitor(node);
   }
 }