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

pkg/analysis_server/lib/src/services/correction/assist_internal.dart

Index: pkg/analysis_server/lib/src/services/correction/assist_internal.dart
diff --git a/pkg/analysis_server/lib/src/services/correction/assist_internal.dart b/pkg/analysis_server/lib/src/services/correction/assist_internal.dart
index 75d52f191d4df158d6f85fd2d3e54a5cec566e8a..989968174a3023da781e5f66ea40df12f0074ea2 100644
--- a/pkg/analysis_server/lib/src/services/correction/assist_internal.dart
+++ b/pkg/analysis_server/lib/src/services/correction/assist_internal.dart
@@ -87,6 +87,12 @@ class AssistProcessor {
   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) {