I noticed that we were pretty inconsistent with the way that we checked the value of clipBehavior in the framework, so I normalized the usages and updated docs where necessary.

This is a breaking change if you used to pass null explicitly to FlatButton, OutlineButton or RaisedButton constructors, expecting to get Clip.none. It will now assert if you do that. Existing implementations that pass null implicitly by not specifying clipBehavior won't need to change their call sites. It always implicitly defaulted to Clip.none before, and it will continue to do that, it's only places where it was explicitly set to null in order to get the implicit default that it will fail.