Use null-aware operators to clean up the code

pkg/analyzer/lib/src/generated/error.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.error;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart' show AstNode;
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 3129 matching lines @@
     this._source = _defaultSource;
   }
 
   Source get source => _source;
 
   /**
    * Set the source to be used when reporting errors to the given [source].
    * Setting the source to `null` will cause the default source to be used.
    */
   void set source(Source source) {
-    this._source = source == null ? _defaultSource : source;
+    this._source = source ?? _defaultSource;
   }
 
   /**
    * Creates an error with properties with the given [errorCode] and
    * [arguments]. The [node] is used to compute the location of the error.
    */
   AnalysisErrorWithProperties newErrorWithProperties(
           ErrorCode errorCode, AstNode node, List<Object> arguments) =>
       new AnalysisErrorWithProperties(
           _source, node.offset, node.length, errorCode, arguments);
@@ skipping 2648 matching lines @@
    * Initialize a newly created error code to have the given [name].
    */
   const TodoCode(String name) : super(name, "{0}");
 
   @override
   ErrorSeverity get errorSeverity => ErrorSeverity.INFO;
 
   @override
   ErrorType get type => ErrorType.TODO;
 }