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

pkg/analysis_server/test/services/refactoring/extract_local_test.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 test.services.refactoring.extract_local;
 
 import 'dart:async';
 import 'dart:convert';
 
 import 'package:analysis_server/plugin/protocol/protocol.dart';
@@ skipping 942 matching lines @@
     _createRefactoringForString('1 + 2');
     // apply refactoring
     return _assertSuccessfulRefactoring('''
 main() {
   var res = 1 + 2;
   int a = res + 3 + 4;
 }
 ''');
   }
 
+  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() {
   int a = 12 + 345;
 }
 ''');
     _createRefactoringForString('12 +');
     // apply refactoring
     return _assertSuccessfulRefactoring('''
 main() {
@@ skipping 113 matching lines @@
     this.refactoringChange = refactoringChange;
     assertTestChangeResult(expectedCode);
   }
 
   void _createRefactoring(int offset, int length) {
     refactoring = new ExtractLocalRefactoring(testUnit, offset, length);
     refactoring.name = 'res';
   }
 
   /**
+   * 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.
    */
   void _createRefactoringForString(String search) {
     int offset = findOffset(search);
     int length = search.length;
     _createRefactoring(offset, length);
   }
 
   void _createRefactoringWithSuffix(String selectionSearch, String suffix) {
     int offset = findOffset(selectionSearch + suffix);
     int length = selectionSearch.length;
     _createRefactoring(offset, length);
   }
 
   List<String> _getCoveringExpressions() {
     List<String> subExpressions = <String>[];
     for (int i = 0; i < refactoring.coveringExpressionOffsets.length; i++) {
       int offset = refactoring.coveringExpressionOffsets[i];
       int length = refactoring.coveringExpressionLengths[i];
       subExpressions.add(testCode.substring(offset, offset + length));
     }
     return subExpressions;
   }
 }