[gardening] Read nnbdMode parameter instead of field on null options.

Fixes crash introduced by https://github.com/dart-lang/sdk/commit/72e559929f7c6db5f38a9b2ad52df67aec624dfc.

Change-Id: Ie9e3cdbcd282a840dcd028661fa93e49d207d38f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/149421
Commit-Queue: Aske Simon Christensen <askesc@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>
Auto-Submit: Aske Simon Christensen <askesc@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>

https://dart.googlesource.com/sdk/+/1ad89b85a2ee82e42c3b522069208892f5e7a1e7
diff --git a/commits.json b/commits.json
index d20483f..628d46b 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "external/github.com/flutter/engine":"89cf074d37633e030bb86a047f6e5bb49ef44b43",
   "external/github.com/flutter/flutter":"a1636b6fef2ebe7840ba6dc1b3f9d77095ef3b45",
-  "sdk":"bbf34035dd984d7dedbcd8a53bc3c79a25b842e8"
+  "sdk":"1ad89b85a2ee82e42c3b522069208892f5e7a1e7"
 }
\ No newline at end of file