Fix parser bug found while trying to reproduce a different bug

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';
 import 'package:analyzer/dart/ast/standard_ast_factory.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/error/error.dart';
 import 'package:analyzer/error/listener.dart';
 import 'package:analyzer/src/dart/ast/ast.dart';
 import 'package:analyzer/src/dart/ast/token.dart';
@@ skipping 5493 matching lines @@
    * actually creating a formal parameter list or changing the current token.
    * Return the token following the parameter list that was parsed, or `null`
    * if the given token is not the first token in a valid parameter list.
    *
    * This method must be kept in sync with [parseFormalParameterList].
    */
   Token skipFormalParameterList(Token startToken) {
     if (!_tokenMatches(startToken, TokenType.OPEN_PAREN)) {
       return null;
     }
-    return (startToken as BeginToken).endToken.next;
+    return (startToken as BeginToken).endToken?.next;
   }
 
   /**
    * Parse the portion of a generic function type after the return type,
    * starting at the [startToken], without actually creating a generic function
    * type or changing the current token. Return the token following the generic
    * function type that was parsed, or `null` if the given token is not the
    * first token in a valid generic function type.
    *
    * This method must be kept in sync with
@@ skipping 3070 matching lines @@
     }
   }
 }
 
 /**
  * Instances of this class are thrown when the parser detects that AST has
  * too many nested expressions to be parsed safely and avoid possibility of
  * [StackOverflowError] in the parser or during later analysis.
  */
 class _TooDeepTreeError {}