* Future<void> main

* Future<void>.delayed

* prefer_void_to_Null

* address review comments

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

* Add ability to set collapse mode in flexible space bar

* Add tests to collapse mode in flexible space bar

* Fix minor style nit

* Fix getting collapse mode does not handle unknown values

* Change const key to final in space bar collapse mode test

* Return null collapse padding if mode is unknown