Use null-aware operators to clean up the code

pkg/analyzer/lib/file_system/file_system.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 analyzer.file_system.file_system;
 
 import 'dart:async';
 
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/util/absolute_path.dart';
@@ skipping 200 matching lines @@
   ResourceUriResolver(this._provider);
 
   @override
   Source resolveAbsolute(Uri uri, [Uri actualUri]) {
     if (!_isFileUri(uri)) {
       return null;
     }
     Resource resource =
         _provider.getResource(_provider.pathContext.fromUri(uri));
     if (resource is File) {
-      return resource.createSource(actualUri != null ? actualUri : uri);
+      return resource.createSource(actualUri ?? uri);
     }
     return null;
   }
 
   @override
   Uri restoreAbsolute(Source source) =>
       _provider.pathContext.toUri(source.fullName);
 
   /**
    * Return `true` if the given [uri] is a `file` URI.
    */
   static bool _isFileUri(Uri uri) => uri.scheme == _FILE_SCHEME;
 }