Use null-aware operators to clean up the code

pkg/analyzer/lib/src/generated/parser.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.parser;
 
 import 'dart:collection';
 import "dart:math" as math;
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 8181 matching lines @@
         keyword != null &&
         _tokenMatchesKeyword(keyword, Keyword.VAR)) {
       _reportErrorForToken(ParserErrorCode.VAR_AND_TYPE, keyword);
     }
     List<VariableDeclaration> variables = new List<VariableDeclaration>();
     variables.add(_parseVariableDeclaration());
     while (_matches(TokenType.COMMA)) {
       _advance();
       variables.add(_parseVariableDeclaration());
     }
-    return new VariableDeclarationList(
-        commentAndMetadata != null ? commentAndMetadata.comment : null,
-        commentAndMetadata != null ? commentAndMetadata.metadata : null,
-        keyword,
-        type,
-        variables);
+    return new VariableDeclarationList(commentAndMetadata?.comment,
+        commentAndMetadata?.metadata, keyword, type, variables);
   }
 
   /**
    * Parse a variable declaration statement. The [commentAndMetadata] is the
    * metadata to be associated with the variable declaration statement, or
    * `null` if there is no attempt at parsing the comment and metadata. Return
    * the variable declaration statement that was parsed.
    *
    *     variableDeclarationStatement ::=
    *         variableDeclarationList ';'
@@ skipping 1754 matching lines @@
    */
   const ParserErrorCode(String name, String message, [String correction])
       : super(name, message, correction);
 
   @override
   ErrorSeverity get errorSeverity => ErrorSeverity.ERROR;
 
   @override
   ErrorType get type => ErrorType.SYNTACTIC_ERROR;
 }