Remove null from flex documentation (#158086)

Closes https://github.com/flutter/flutter/issues/158085
https://dart.googlesource.com/external/github.com/flutter/flutter/+/8591d0c16a6c7edfbea3b5b1bf08ae09db117b71

DEPS

diff --git a/DEPS b/DEPS
index 0c5f0e0..2db23ef 100644
--- a/DEPS
+++ b/DEPS
@@ -271,7 +271,7 @@
 
   'engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + '817882c36be242dcffb678d423b0337f6c9598db',
 
-  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + 'ed91ba1ba5bb24121106ff37d7ba7f687c6f0349',
+  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '8591d0c16a6c7edfbea3b5b1bf08ae09db117b71',
 
   '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 50c4077..c1b263c 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "engine/src/flutter":"a215a3e97fa89f719fcb28cc00b693f614be4ed4",
   "engine/src/flutter/third_party/dart":"817882c36be242dcffb678d423b0337f6c9598db",
-  "flutter":"ed91ba1ba5bb24121106ff37d7ba7f687c6f0349"
+  "flutter":"8591d0c16a6c7edfbea3b5b1bf08ae09db117b71"
 }
\ No newline at end of file