Fix lints: unnecessary_brace_in_string_interp

pkg/analysis_server/test/analysis/notification_navigation_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.analysis.notification.navigation;
 
 import 'dart:async';
 
 import 'package:analysis_server/plugin/protocol/protocol.dart';
 import 'package:analysis_server/src/constants.dart';
@@ skipping 26 matching lines @@
     for (NavigationTarget target in testTargets) {
       if (targetFiles[target.fileIndex] == file &&
           target.offset == offset &&
           target.length == length) {
         testTarget = target;
         return;
       }
     }
     fail(
         'Expected to find target (file=$file; offset=$offset; length=$length) in\n'
-        '${testRegion} in\n'
+        '$testRegion in\n'
         '${testTargets.join('\n')}');
   }
 
   void assertHasOperatorRegion(String regionSearch, int regionLength,
       String targetSearch, int targetLength) {
     assertHasRegion(regionSearch, regionLength);
     assertHasTarget(targetSearch, targetLength);
   }
 
   /**
@@ skipping 869 matching lines @@
 
   test_type_void() async {
     addTestFile('''
 void main() {
 }
 ''');
     await prepareNavigation();
     assertNoRegionAt('void');
   }
 }