Make documentaiton comments Null Safe by default, support %language directive instead.

This is consistent with what we do for user project.
Null safety is not an experiment anymore.

OTOH, any future experiments should be applied on top of the latest
language version.

Change-Id: I6dcbc954aefefb1f7a03c9ceaf1d25be28e7adf4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/174488
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

https://dart.googlesource.com/sdk/+/b4052982b5fe2e46babf211f2b0d7c0bf31bbbee
diff --git a/commits.json b/commits.json
index 873cf35..185d49e 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "external/github.com/flutter/engine":"a4ce848dfb6fb002544494e762f53a826830cc1d",
   "external/github.com/flutter/flutter":"a7f5fd5360007518644597c60a1f2169eacccc2b",
-  "sdk":"6c6dba54bd9e9f90beeac1392f61cac2ce2b3701"
+  "sdk":"b4052982b5fe2e46babf211f2b0d7c0bf31bbbee"
 }
\ No newline at end of file