Handle null initializer, this is being hit internally when compiling large
projects.

I don't know the exact situation that is being hit here and if something else
is going on, or continuing here is sane. It does appear to work though.

Change-Id: I7cbdf59f954ed80e0a9c71430b5661a335e21d9b
Reviewed-on: https://dart-review.googlesource.com/c/85402
Reviewed-by: Peter von der Ahé <ahe@google.com>
Commit-Queue: Jake Macdonald <jakemac@google.com>
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart
index b1a15e8..895661e 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart
@@ -1125,6 +1125,7 @@
           return unhandled(
               "null", forwarder.name.name, origin.fileOffset, origin.fileUri);
         }
+        if (originNamed.initializer == null) continue;
         forwarderNamed.initializer = cloner.clone(originNamed.initializer);
         forwarderNamed.initializer.parent = forwarderNamed;
       }