Update version.dart (#3873)

packages/flutter_tools/lib/src/version.dart

diff --git a/packages/flutter_tools/lib/src/version.dart b/packages/flutter_tools/lib/src/version.dart
index 5b402a4..95d999c 100644
--- a/packages/flutter_tools/lib/src/version.dart
+++ b/packages/flutter_tools/lib/src/version.dart
@@ -11,6 +11,7 @@
   'master',
   'alpha',
   'hackathon',
+  'codelab',
   'beta'
 ]);