[dart2js] Clean up APIs that were only used for unsound null safety.

Change-Id: Iff76a964317388c869373fbebe15ab4cbbe84468
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/416522
Reviewed-by: Nate Biggs <natebiggs@google.com>

https://dart.googlesource.com/sdk/+/9d18a99087f507f911b57ef4cd81f8182b5940d8
diff --git a/DEPS b/DEPS
index dee8687..27ecfc4 100644
--- a/DEPS
+++ b/DEPS
@@ -238,7 +238,7 @@
 ]
 
 deps = {
-  'flutter/engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + 'b3805128692a07af6965df04217684c7e6113f68',
+  'flutter/engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + '9d18a99087f507f911b57ef4cd81f8182b5940d8',
 
   'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + 'fd168e11926c9a1b7db91293814bc86c8330c867',
 
diff --git a/commits.json b/commits.json
index b5e2a9d..50ceea9 100644
--- a/commits.json
+++ b/commits.json
@@ -1,4 +1,4 @@
 {
   "flutter":"fd168e11926c9a1b7db91293814bc86c8330c867",
-  "flutter/engine/src/flutter/third_party/dart":"b3805128692a07af6965df04217684c7e6113f68"
+  "flutter/engine/src/flutter/third_party/dart":"9d18a99087f507f911b57ef4cd81f8182b5940d8"
 }
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