Fix lints: unnecessary_brace_in_string_interp

pkg/analysis_server/lib/src/utilities/average.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 analysis_server.src.utilities.average;
 
 /// Simple rolling average sample counter.
 class Average {
   num _val;
   final int _sampleCount;
 
   /// Create an average with the given (optional) sample count size.
   Average([this._sampleCount = 20]);
 
   /// The current average.
   num get value => _val ?? 0;
 
   /// Add the given [sample].
   void addSample(num sample) {
     if (_val == null) {
       _val = sample;
     } else {
       _val = _val * ((_sampleCount - 1) / _sampleCount) +
           sample * (1 / _sampleCount);
     }
   }
 
   @override
-  String toString() => 'average: ${value}';
+  String toString() => 'average: $value';
 }