update analysis options for nnbd (#3319)
update analysis options for nnbd (#3319)

With NNBD assert(foo != null) for a non nullable foo generates an analysis error.
However as long as we support mixed-mode we want these asserts it, so disabling the check.
flutter/plugins,mehmetf/plugins,flutter/plugins,flutter/plugins,flutter/plugins,flutter/plugins,flutter/plugins,flutter/plugins,tvolkert/plugins,flutter/plugins,mehmetf/plugins,tvolkert/plugins,mehmetf/plugins,tvolkert/plugins,mehmetf/plugins,mehmetf/plugins,tvolkert/plugins