Issue 25251. Fix for extracting string literal part with zero length.

pkg/analysis_server/test/services/refactoring/extract_local_test.dart

Index: pkg/analysis_server/test/services/refactoring/extract_local_test.dart
diff --git a/pkg/analysis_server/test/services/refactoring/extract_local_test.dart b/pkg/analysis_server/test/services/refactoring/extract_local_test.dart
index cae09e7999dc7128f9fff5aafacddb4c519d4db7..9903a1d59bd20ba49856cc1c837adffe57311329 100644
--- a/pkg/analysis_server/test/services/refactoring/extract_local_test.dart
+++ b/pkg/analysis_server/test/services/refactoring/extract_local_test.dart
@@ -960,6 +960,22 @@ main() {
 ''');
   }
 
+  test_singleExpression_string() {
+    indexTestUnit('''
+void main() {
+  print("1234");
+}
+''');
+    _createRefactoringAtString('34"');
+    // apply refactoring
+    return _assertSuccessfulRefactoring('''
+void main() {
+  var res = "1234";
+  print(res);
+}
+''');
+  }
+
   test_singleExpression_trailingNotWhitespace() {
     indexTestUnit('''
 main() {
@@ -1093,6 +1109,16 @@ main() {
   }
 
   /**
+   * Creates a new refactoring in [refactoring] at the offset of the given
+   * [search] pattern, and with the length `0`.
+   */
+  void _createRefactoringAtString(String search) {
+    int offset = findOffset(search);
+    int length = 0;
+    _createRefactoring(offset, length);
+  }
+
+  /**
    * Creates a new refactoring in [refactoring] for the selection range of the
    * given [search] pattern.
    */