Use null-aware operators to clean up the code

pkg/analyzer/lib/src/generated/source.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.src.generated.source;
 
 import 'dart:collection';
 import "dart:math" as math;
 
 import 'package:analyzer/file_system/file_system.dart';
@@ skipping 94 matching lines @@
 
   @override
   Source resolveAbsolute(Uri uri, [Uri actualUri]) {
     String mapping = _urlMappings[uri.toString()];
     if (mapping == null) return null;
 
     Uri fileUri = new Uri.file(mapping);
     if (!fileUri.isAbsolute) return null;
 
     JavaFile javaFile = new JavaFile.fromUri(fileUri);
-    return new FileBasedSource(javaFile, actualUri != null ? actualUri : uri);
+    return new FileBasedSource(javaFile, actualUri ?? uri);
   }
 }
 
 /**
  * Instances of the class `DartUriResolver` resolve `dart` URI's.
  */
 class DartUriResolver extends UriResolver {
   /**
    * The name of the `dart` scheme.
    */
@@ skipping 804 matching lines @@
   Source resolveAbsolute(Uri uri, [Uri actualUri]);
 
   /**
    * Return an absolute URI that represents the given [source], or `null` if a
    * valid URI cannot be computed.
    *
    * The computation should be based solely on [source.fullName].
    */
   Uri restoreAbsolute(Source source) => null;
 }