Use null-aware operators to clean up the code

pkg/analyzer/lib/file_system/memory_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.memory_file_system;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:core' hide Resource;
 
@@ skipping 170 matching lines @@
     _notifyWatchers(newPath, ChangeType.ADD);
     return newFile;
   }
 
   File updateFile(String path, String content, [int stamp]) {
     path = pathContext.normalize(path);
     newFolder(pathContext.dirname(path));
     _MemoryFile file = new _MemoryFile(this, path);
     _pathToResource[path] = file;
     _pathToContent[path] = content;
-    _pathToTimestamp[path] = stamp != null ? stamp : nextStamp++;
+    _pathToTimestamp[path] = stamp ?? nextStamp++;
     _notifyWatchers(path, ChangeType.MODIFY);
     return file;
   }
 
   void _checkFileAtPath(String path) {
     _MemoryResource resource = _pathToResource[path];
     if (resource is! _MemoryFile) {
       throw new ArgumentError(
           'File expected at "$path" but ${resource.runtimeType} found');
     }
@@ skipping 368 matching lines @@
   bool operator ==(other) {
     if (runtimeType != other.runtimeType) {
       return false;
     }
     return path == other.path;
   }
 
   @override
   String toString() => path;
 }