Revert "Roll src/third_party/dart 6e2b3f0326..b99df2be86 (5 commits) (#11287)" (#11297)

This reverts commit df31dba49f0a23bafb34b4f614074e991ee813a7.
https://dart.googlesource.com/external/github.com/flutter/engine/+/43db5257f46badb676047a5c1901c059fae63088
diff --git a/commits.json b/commits.json
index 73449b8..cd6c394 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
-  "external/github.com/flutter/engine":"df31dba49f0a23bafb34b4f614074e991ee813a7",
+  "external/github.com/flutter/engine":"43db5257f46badb676047a5c1901c059fae63088",
   "external/github.com/flutter/flutter":"700020fc6bd08f373042b9d1d38747c9b2a32d8c",
   "sdk":"3865187956cad51bc7065d30beb8ab89702b7953"
 }
\ No newline at end of file