Use null-aware operators to clean up the code

pkg/analyzer/lib/src/generated/incremental_resolver.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.incremental_resolver;
 
 import 'dart:collection';
 import 'dart:math' as math;
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 1704 matching lines @@
   void _setPrecedingComments(Token token, CommentToken comment) {
     if (token is BeginTokenWithComment) {
       token.precedingComments = comment;
     } else if (token is KeywordTokenWithComment) {
       token.precedingComments = comment;
     } else if (token is StringTokenWithComment) {
       token.precedingComments = comment;
     } else if (token is TokenWithComment) {
       token.precedingComments = comment;
     } else {
-      Type parentType = token != null ? token.runtimeType : null;
+      Type parentType = token?.runtimeType;
       throw new AnalysisException('Uknown parent token type: $parentType');
     }
   }
 
   void _shiftTokens(Token token) {
     while (token != null) {
       if (token.offset > _updateOffset) {
         token.offset += _updateDelta;
       }
       // comments
@@ skipping 501 matching lines @@
   @override
   String toString() => name;
 }
 
 class _TokenPair {
   final _TokenDifferenceKind kind;
   final Token oldToken;
   final Token newToken;
   _TokenPair(this.kind, this.oldToken, this.newToken);
 }