Additional static type checking

[bwilkerson]

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

• Implement a set of "strict" static analysis rules sdk#33749
• I would like a --strict-await static analysis option (2.1+) sdk#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)?

[matanlurey]

Note also dart-lang/sdk#34304, my attempt to gather information.