update keyword list to include "late" modifier

Change-Id: I10d815d78d8198749680af1b56eff8eb31a6f895
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/99800
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Dan Rubel <danrubel@google.com>
diff --git a/pkg/front_end/lib/src/scanner/token.dart b/pkg/front_end/lib/src/scanner/token.dart
index c1989c8..0ffd35b 100644
--- a/pkg/front_end/lib/src/scanner/token.dart
+++ b/pkg/front_end/lib/src/scanner/token.dart
@@ -318,6 +318,7 @@
     IN,
     INTERFACE,
     IS,
+    LATE,
     LIBRARY,
     MIXIN,
     NATIVE,