Consider using Object as type argument for generic parameters in place of dynamic

[natebosch]

This is mainly coming up because of how it interacts with --no-implicit-dynamic. We're considering enabling that flag in more places because it's helping us catch bugs - but it also interacts poorly with signatures that take generic classes with <dynamic>

I'm not sure how many APIs are impacted here but we can take one example: the Set.from constructor:

As it is, Set.from(Iterable elements) there is an implicit dynamic error in new Set<String>.from(['foo']);. If instead the signature were Set.from(Iterable<Object> elements) this would be error free.

cc @leafpetersen