[DEPS] Rev package:protobuf

Change-Id: I4031e517d0700242c17b4c6b117a17b53b52caa8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/418620
Commit-Queue: Derek Xu <derekx@google.com>
Reviewed-by: Ben Konyi <bkonyi@google.com>

https://dart.googlesource.com/sdk/+/21f1620974c291cd636b728a236c600cb65bf9f6
diff --git a/DEPS b/DEPS
index 0a9db38..1408a75 100644
--- a/DEPS
+++ b/DEPS
@@ -238,7 +238,7 @@
 ]
 
 deps = {
-  'flutter/engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + 'a7615e0506d4d0c3572d791006ef33c9e4368d54',
+  'flutter/engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + '21f1620974c291cd636b728a236c600cb65bf9f6',
 
   'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '228a011707242d8e20cedc25a3825b08b30b3a46',
 
diff --git a/commits.json b/commits.json
index 4c2ef24..8db692e 100644
--- a/commits.json
+++ b/commits.json
@@ -1,4 +1,4 @@
 {
   "flutter":"228a011707242d8e20cedc25a3825b08b30b3a46",
-  "flutter/engine/src/flutter/third_party/dart":"a7615e0506d4d0c3572d791006ef33c9e4368d54"
+  "flutter/engine/src/flutter/third_party/dart":"21f1620974c291cd636b728a236c600cb65bf9f6"
 }
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