Use null-aware operators to clean up the code

pkg/analyzer/lib/src/dart/ast/utilities.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.dart.ast.utilities;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 3661 matching lines @@
    */
   AstNode _foundNode;
 
   /**
    * Initialize a newly created locator to locate an [AstNode] by locating the
    * node within an AST structure that corresponds to the given range of
    * characters (between the [startOffset] and [endOffset] in the source.
    */
   NodeLocator(int startOffset, [int endOffset])
       : this._startOffset = startOffset,
-        this._endOffset = endOffset == null ? startOffset : endOffset;
+        this._endOffset = endOffset ?? startOffset;
 
   /**
    * Return the node that was found that corresponds to the given source range
    * or `null` if there is no such node.
    */
   AstNode get foundNode => _foundNode;
 
   /**
    * Search within the given AST [node] for an identifier representing an
    * element in the specified source range. Return the element that was found,
@@ skipping 83 matching lines @@
 
   /**
    * Initialize a newly created locator to locate the deepest [AstNode] for
    * which `node.offset <= [startOffset]` and `[endOffset] < node.end`.
    *
    * If [endOffset] is not provided, then it is considered the same as the
    * given [startOffset].
    */
   NodeLocator2(int startOffset, [int endOffset])
       : this._startOffset = startOffset,
-        this._endOffset = endOffset == null ? startOffset : endOffset;
+        this._endOffset = endOffset ?? startOffset;
 
   /**
    * Search within the given AST [node] and return the node that was found,
    * or `null` if no node was found.
    */
   AstNode searchWithin(AstNode node) {
     if (node == null) {
       return null;
     }
     try {
@@ skipping 3986 matching lines @@
    * Safely visit the given [token], printing the [suffix] after the token if it
    * is non-`null`.
    */
   void _visitTokenWithSuffix(Token token, String suffix) {
     if (token != null) {
       _writer.print(token.lexeme);
       _writer.print(suffix);
     }
   }
 }