Fix exception when parsing broken code (issue 29447)

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 58571133ee7eab58999afd80b9c14dd00fb58ff3..96326ebd71bfa17720a10d4ee6b2672989a527d2 100644
--- a/pkg/analyzer/lib/src/generated/parser.dart
+++ b/pkg/analyzer/lib/src/generated/parser.dart
@@ -5535,6 +5535,9 @@ class Parser {
     Token next = startToken.next; // Skip 'Function'
     if (_tokenMatches(next, TokenType.LT)) {
       next = skipTypeParameterList(next);
+      if (next == null) {
+        return null;
+      }
     }
     return skipFormalParameterList(next);
   }