Use null aware operators to clean up code

pkg/analysis_server/lib/src/server/driver.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 driver;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:math';
 
@@ skipping 316 matching lines @@
   HttpAnalysisServer httpServer;
 
   StdioAnalysisServer stdioServer;
 
   Driver();
 
   /**
    * Set the [plugins] that are defined outside the analysis_server package.
    */
   void set userDefinedPlugins(List<Plugin> plugins) {
-    _userDefinedPlugins = plugins == null ? <Plugin>[] : plugins;
+    _userDefinedPlugins = plugins ?? <Plugin>[];
   }
 
   /**
    * Use the given command-line [arguments] to start this server.
    */
   void start(List<String> arguments) {
     CommandLineParser parser = _createArgParser();
     ArgResults results = parser.parse(arguments, <String, String>{});
     if (results[HELP_OPTION]) {
       _printUsage(parser.parser);
@@ skipping 263 matching lines @@
    */
   static void _rollLogFiles(String path, int numOld) {
     for (int i = numOld - 1; i >= 0; i--) {
       try {
         String oldPath = i == 0 ? path : '$path.$i';
         new File(oldPath).renameSync('$path.${i+1}');
       } catch (e) {}
     }
   }
 }