They are causing nothing but trouble. We should fix these before turning them back on.

See https://github.com/flutter/flutter/issues/9986 https://github.com/flutter/flutter/issues/9987

* Rev versions for alpha branch

* Rev versions after alpha

* is default

* hit target

* correct test name

* review note

Also, fix some logic in SliverPadding.

Fixes https://github.com/flutter/flutter/issues/9954

* enable always_require_non_null_named_parameters lint

* Update home.dart

Also includes minor doc fixes.

Also fixes a small typo.

And add a script to rev the versions.

Turns out that `[]:` means something in markdown, so you have to
escape the `:` with a backslash.

It wasn't tested, which is why it didn't work before.

Now that we can host packages on pub that depend on Flutter, we can host
flutter_markdown in its own git repository.

Corrects type from string introduced in
21712a49.

Fixes https://github.com/flutter/flutter/issues/9929