Add missing localization generation for widgets (#169695)

Adds missing localization generation for widget translations
I ran into this issue when I was trying to generate files for
https://github.com/flutter/flutter/pull/169150/files, and found out the
files were being generated for cupertino and material but not widgets.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.
https://dart.googlesource.com/external/github.com/flutter/flutter/+/538cf22b635888fc7df0a424fd7b55c9b01edf41

DEPS

diff --git a/DEPS b/DEPS
index a063d75..00f8446 100644
--- a/DEPS
+++ b/DEPS
@@ -237,7 +237,7 @@
 deps = {
   'flutter/engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + '0467dfc43458e03624d93dba35e819a462f2def9',
 
-  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '4cc47e8d52ad96a9c70d2eef839665d8c858d50e',
+  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '538cf22b635888fc7df0a424fd7b55c9b01edf41',
 
   '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 131f1e5..b8d2d78 100644
--- a/commits.json
+++ b/commits.json
@@ -1,4 +1,4 @@
 {
-  "flutter":"4cc47e8d52ad96a9c70d2eef839665d8c858d50e",
+  "flutter":"538cf22b635888fc7df0a424fd7b55c9b01edf41",
   "flutter/engine/src/flutter/third_party/dart":"0467dfc43458e03624d93dba35e819a462f2def9"
 }
\ No newline at end of file