Fix for extracting from interpolated string.

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 'package:analysis_server/src/protocol.dart';
 import 'package:analysis_server/src/services/refactoring/extract_local.dart';
@@ skipping 893 matching lines @@
     _createRefactoringForString("'abc'");
     // apply refactoring
     return _assertSuccessfulRefactoring('''
 main() {
   var res = 'abc';
   print(res);
 }
 ''');
   }
 
+  test_stringLiteralPart() {
+    indexTestUnit(r'''
+main() {
+  int x = 1;
+  int y = 2;
+  print('$x+$y=${x+y}');
+}
+''');
+    _createRefactoringForString(r'$x+$y');
+    // apply refactoring
+    return _assertSuccessfulRefactoring(r'''
+main() {
+  int x = 1;
+  int y = 2;
+  var res = '$x+$y';
+  print('${res}=${x+y}');
+}
+''');
+  }
+
   Future _assertInitialConditions_fatal_selection() {
     return refactoring.checkInitialConditions().then((status) {
       assertRefactoringStatus(
           status,
           RefactoringProblemSeverity.FATAL,
           expectedMessage: 'Expression must be selected to activate this refactoring.');
     });
   }
 
   /**
@@ skipping 23 matching lines @@
     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);
   }
 }