[engine] add back opt out for merged threads. (#55952)

Add back opt out for merged threads for customer money.
https://dart.googlesource.com/external/github.com/flutter/engine/+/d0fd04a73d2c5fd6fd29a2a27e7740b3be8ff124

DEPS

diff --git a/DEPS b/DEPS
index 4672e56..e34dc51 100644
--- a/DEPS
+++ b/DEPS
@@ -279,7 +279,7 @@
 deps = {
   'engine/src': 'https://github.com/flutter/buildroot.git' + '@' + 'c8f93f25a19cefaaeb64d4323e2fc8c9ccd20479',
 
-  'engine/src/flutter': Var('flutter_git') + '/mirrors/engine' + '@' + '42c2a40ee1702eac86effa46187a83dc29f53adc',
+  'engine/src/flutter': Var('flutter_git') + '/mirrors/engine' + '@' + 'd0fd04a73d2c5fd6fd29a2a27e7740b3be8ff124',
 
   'engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + '6adcc1ca54cc53b20555eaf5b9b9ad41abb3a044',
 

commits.json

diff --git a/commits.json b/commits.json
index c5bebbf..a4e2f35 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
-  "engine/src/flutter":"42c2a40ee1702eac86effa46187a83dc29f53adc",
+  "engine/src/flutter":"d0fd04a73d2c5fd6fd29a2a27e7740b3be8ff124",
   "engine/src/flutter/third_party/dart":"6adcc1ca54cc53b20555eaf5b9b9ad41abb3a044",
   "flutter":"88816add42666e2de88c4bffffa5fadf4cfcb9ab"
 }
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