Additional static type checking

There has been a lot of discussion around the old strong mode options 'declaration-casts', 'implicit-casts', and 'implicit-dynamic'. The discussions tend to be of two forms: what are the right set of options going forward, and are these language variants or just lints. As examples of the discussions, see
- https://github.com/dart-lang/sdk/issues/33749
- https://github.com/dart-lang/sdk/issues/33823

While I would value input from the language team on the first question, I created this issue to request a decision on the second question. Should these options be supported by all of our tooling, or should they only be supported by the analyzer (probably as lints)?