Fix null checks in _BundleWriterReferences.

Change-Id: I9c99f62846cbc574e1a4694c5b4bb18c578ab6e2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/189422
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

https://dart.googlesource.com/sdk/+/637d4f84076faecbbf87c03e943a1bdce2ebe93f
diff --git a/commits.json b/commits.json
index 4167a98..2f7188e 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "external/github.com/flutter/engine":"09b2468785a3e07132cac1fba99b1beafdf9dcec",
   "external/github.com/flutter/flutter":"a0ba6464089f906fefbf610732b2c9a7b66a1260",
-  "sdk":"e26bd4471ecc6f5fffdf8883bcf4b946d5baed1d"
+  "sdk":"637d4f84076faecbbf87c03e943a1bdce2ebe93f"
 }
\ No newline at end of file