Fix lints: unnecessary_brace_in_string_interp

pkg/analysis_server/benchmark/perf/completion_timing_tests.dart

 // Copyright (c) 2015, 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 server.performance.analysis.timing;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:args/args.dart';
@@ skipping 60 matching lines @@
       timings.add(new Duration(milliseconds: stopwatch.elapsed.inMilliseconds));
     });
 
     setAnalysisRoot();
     sendAnalysisSetPriorityFiles([priorityFile]);
     sendCompletionGetSuggestions(priorityFile, offset);
 
     await analysisFinished;
 
     print('analysis completed in ${stopwatch.elapsed}');
-    print('completion received at : ${timings}');
+    print('completion received at : $timings');
     await shutdown();
   }
 }