add missing `await`s (#144978)

This was uncovered as part of https://github.com/flutter/flutter/pull/144706.
https://dart.googlesource.com/external/github.com/flutter/flutter/+/e277549751cd64b89ac278d187a67c0b94a786a5

DEPS

diff --git a/DEPS b/DEPS
index 050701b..cd1fd2b 100644
--- a/DEPS
+++ b/DEPS
@@ -290,7 +290,7 @@
 
   'engine/src/third_party/dart': Var('dart_git') + '/sdk' + '@' + 'af700baa49b9fdab170ff8ed2b4b9fe26772f48b',
 
-  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '46cb2d7b013cd356962ed41d55fba2e65ddb89f2',
+  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + 'e277549751cd64b89ac278d187a67c0b94a786a5',
 
   'engine/src/flutter/third_party/depot_tools':
   Var('chromium_git') + '/chromium/tools/depot_tools.git' + '@' + '580b4ff3f5cd0dcaa2eacda28cefe0f45320e8f7',

commits.json

diff --git a/commits.json b/commits.json
index ad69463..0549cbd 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "engine/src/flutter":"12a6da3f6cbe82501ddff286a9573f04bdd8e3e5",
   "engine/src/third_party/dart":"af700baa49b9fdab170ff8ed2b4b9fe26772f48b",
-  "flutter":"46cb2d7b013cd356962ed41d55fba2e65ddb89f2"
+  "flutter":"e277549751cd64b89ac278d187a67c0b94a786a5"
 }
\ No newline at end of file