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

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

Index: pkg/analysis_server/lib/src/services/correction/fix_internal.dart
diff --git a/pkg/analysis_server/lib/src/services/correction/fix_internal.dart b/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
index 39f96e73dcaf0b93c94927ac57fbe28b75bb913a..1a99f8fa905819498be3a5fbcdf9c29feeb1cb3f 100644
--- a/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
+++ b/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
@@ -139,6 +139,12 @@ class FixProcessor {
   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) {