Fix parser bug found while trying to reproduce a different bug

pkg/analyzer/lib/src/generated/parser.dart

Index: pkg/analyzer/lib/src/generated/parser.dart
diff --git a/pkg/analyzer/lib/src/generated/parser.dart b/pkg/analyzer/lib/src/generated/parser.dart
index 94baa56626a2da57b65f9a56c7906cb16dd3ac50..e6e52439105b0167c9802c9fad03abb9398740b5 100644
--- a/pkg/analyzer/lib/src/generated/parser.dart
+++ b/pkg/analyzer/lib/src/generated/parser.dart
@@ -2,8 +2,6 @@
 // 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;
 
@@ -5517,7 +5515,7 @@ class Parser {
     if (!_tokenMatches(startToken, TokenType.OPEN_PAREN)) {
       return null;
     }
-    return (startToken as BeginToken).endToken.next;
+    return (startToken as BeginToken).endToken?.next;
   }
 
   /**