[ddc] Fix copy paste error in ddb script

Change-Id: I17e8c72369a5b9331dcceaa29343f6596e7440b1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/153821
Auto-Submit: Nicholas Shahan <nshahan@google.com>
Reviewed-by: Jonas Termansen <sortie@google.com>
Commit-Queue: Jonas Termansen <sortie@google.com>

pkg/dev_compiler/tool/ddb

@@ -347,7 +347,7 @@ try {
   if ($nnbd) {
     dart.nullSafety($soundNullSafety);
     dart.weakNullSafetyWarnings(!$soundNullSafety);
-    sdk.dart.nonNullAsserts($nonNullAsserts);
+    dart.nonNullAsserts($nonNullAsserts);
   }
   _isolate_helper.startRootIsolate(() => {}, []);
   main();