Fix lints: unnecessary_brace_in_string_interp

pkg/analysis_server/lib/src/services/refactoring/refactoring_internal.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;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analysis_server/src/protocol_server.dart' hide Element;
@@ skipping 83 matching lines @@
   }
 
   /**
    * Returns the [SourceEdit] to replace this reference.
    */
   SourceEdit createEdit(String newText, {String id}) {
     return newSourceEdit_range(range, newText, id: id);
   }
 
   @override
-  String toString() => '${file}@${range}';
+  String toString() => '$file@$range';
 }