enable `strict-casts`

[goderbauer]

Helps in catching unintended runtime errors related to dynamic.

See also #750, which is the other part of this puzzle.

[lrhn]

The "strict-casts" flag is not a lint, just an analyzer flag (https://dart.dev/guides/language/analysis-options#enabling-additional-type-checks).

It makes it an error to have an implicit downcast from dynamic. That's one of the affordances of dynamic, but if someone wants to disable it, they're free to do so.

I don't think it's something we should enabled by default in the recommended lints, not unless we would also be willing to work towards removing it from the language entirely. I don't think we're there today.

[devoncarew]

We need more investigation to take action here, and will likely evaluate strict-inference and strict-raw-types as well.

[devoncarew]

From a discussion with @leafpetersen, he thought this was likely worth adding, if we can handle the special case for common patterns in web code.

[lrhn]

If we change the language to give another easy way to get a general downcast, or an explicit dynamic section of code where the mode won't apply, or specify specific workaround patterns, then it's OK to discourage uses that don't use those patterns. We have not done that yet.
I still don't think we're ready to effectively remove implicit downcasts from dynamic from the language.
We need an opt-in syntax, and preferably one which isn't too cumbersome, and which works correctly for something like for (int x in listOfDouble as dynamic).