domain: Remove DefaultsContext.get and use return value from .push.
features: Re-codegen.
migrations: Add overridable defaultXxx methods in Builders.
features: Re-codegen.
util: Add Argument.arg(Class, String) overload.
migrations: Add defaults(DefaultsContext) overload to reduce need to push/pop.
features: Re-codegen.
domain: Make DefaultsContext.pop an instance method.