Fix exception when parsing broken code (issue 29447)

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 5517 matching lines @@
    * 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
    * [parseGenericFunctionTypeAfterReturnType].
    */
   Token skipGenericFunctionTypeAfterReturnType(Token startToken) {
     Token next = startToken.next; // Skip 'Function'
     if (_tokenMatches(next, TokenType.LT)) {
       next = skipTypeParameterList(next);
+      if (next == null) {
+        return null;
+      }
     }
     return skipFormalParameterList(next);
   }
 
   /**
    * Parse a prefixed identifier, starting at the [startToken], without actually
    * creating a prefixed identifier or changing the current token. Return the
    * token following the prefixed identifier that was parsed, or `null` if the
    * given token is not the first token in a valid prefixed identifier.
    *
@@ skipping 3052 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 {}