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

pkg/analysis_server/lib/src/services/correction/fix_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 analysis_server.src.services.correction.fix_internal;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:core';
 
@@ skipping 121 matching lines @@
   }
 
   DartType get coreTypeBool => _getCoreType('bool');
 
   /**
    * Returns the EOL to use for this [CompilationUnit].
    */
   String get eol => utils.endOfLine;
 
   Future<List<Fix>> compute() async {
+    // If the source was changed between the constructor and running
+    // this asynchronous method, it is not safe to use the unit.
+    if (context.getModificationStamp(unitSource) != fileStamp) {
+      return const <Fix>[];
+    }
+
     try {
       utils = new CorrectionUtils(unit);
     } catch (e) {
       throw new CancelCorrectionException(exception: e);
     }
 
     errorOffset = error.offset;
     errorLength = error.length;
     errorEnd = errorOffset + errorLength;
     errorRange = new SourceRange(errorOffset, errorLength);
@@ skipping 2793 matching lines @@
       }
     }
   }
 
   void _updateList(Iterable<Element> elements) {
     for (Element element in elements) {
       _update(element);
     }
   }
 }