Make sure that a Checkbox doesn't crash in 0x0 environment (#173178)

This is my attempt to handle
https://github.com/flutter/flutter/issues/6537 for the Checkbox UI
control.
https://dart.googlesource.com/external/github.com/flutter/flutter/+/a9be1dfab3a3ca0e31190a9ec23c2e96fdd64ff1

DEPS

diff --git a/DEPS b/DEPS
index 466143f..e1bca82 100644
--- a/DEPS
+++ b/DEPS
@@ -231,7 +231,7 @@
 deps = {
   'flutter/engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + '53030709a500bee517d3a7fa960df9dc188c1287',
 
-  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '27346436423c8e90840f1775e45cdcbd48ad6958',
+  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + 'a9be1dfab3a3ca0e31190a9ec23c2e96fdd64ff1',
 
   'flutter/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 7395cb6..f907c84 100644
--- a/commits.json
+++ b/commits.json
@@ -1,4 +1,4 @@
 {
-  "flutter":"27346436423c8e90840f1775e45cdcbd48ad6958",
+  "flutter":"a9be1dfab3a3ca0e31190a9ec23c2e96fdd64ff1",
   "flutter/engine/src/flutter/third_party/dart":"53030709a500bee517d3a7fa960df9dc188c1287"
 }
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