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

pkg/analysis_server/lib/src/services/refactoring/extract_local.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 services.src.refactoring.extract_local;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analysis_server/src/protocol_server.dart' hide Element;
@@ skipping 263 matching lines @@
     }
     // we need enclosing block to add variable declaration statement
     if (coveringNode == null ||
         coveringNode.getAncestor((node) => node is Block) == null) {
       return new RefactoringStatus.fatal(
           'Expression inside of function must be selected '
           'to activate this refactoring.');
     }
     // part of string literal
     if (coveringNode is StringLiteral) {
-      if (selectionRange.offset > coveringNode.offset &&
+      if (selectionRange.length != 0 &&
+          selectionRange.offset > coveringNode.offset &&
           selectionRange.end < coveringNode.end) {
         stringLiteralPart = selectionStr;
         return new RefactoringStatus();
       }
     }
     // single node selected
     if (rootExpression != null) {
       singleExpression = rootExpression;
       selectionRange = rangeNode(singleExpression);
       wholeStatementExpression = singleExpression.parent is ExpressionStatement;
@@ skipping 365 matching lines @@
 
   _TokenLocalElementVisitor(this.map);
 
   visitSimpleIdentifier(SimpleIdentifier node) {
     Element element = node.staticElement;
     if (element is LocalVariableElement) {
       map[node.token] = element;
     }
   }
 }