Use null aware operators to clean up code

pkg/analysis_server/lib/src/services/completion/completion_performance.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.completion.manager;
 
 import 'package:analyzer/src/generated/source.dart';
 
 /**
  * Overall performance of a code completion operation.
@@ skipping 24 matching lines @@
   String get startTimeAndMs => '${start.millisecondsSinceEpoch} - $start';
 
   String get suggestionCount {
     if (notificationCount < 1) return '';
     if (notificationCount == 1) return '$suggestionCountFirst';
     return '$suggestionCountFirst,  $suggestionCountLast';
   }
 
   void complete([String tag = null]) {
     _stopwatch.stop();
-    _logDuration(tag != null ? tag : 'total time', _stopwatch.elapsed);
+    _logDuration(tag ?? 'total time', _stopwatch.elapsed);
   }
 
   void logElapseTime(String tag) {
     Duration end = _stopwatch.elapsed;
     Duration start = _startTimes[tag];
     if (start == null) {
       _logDuration(tag, null);
       return null;
     }
     _logDuration(tag, end - start);
@@ skipping 54 matching lines @@
    */
   final String name;
 
   /**
    * The elapse time or `null` if undefined.
    */
   final Duration elapsed;
 
   OperationPerformance(this.name, this.elapsed);
 }