Finalize override semantics with dynamic-typed parameters

[vsmenon]

We need to nail down which of these overrides to allow and disallow:

class A {
  void foo(dynamic x) { ... }
  void bar(int y) { ... }
}

class B extends A {
  void foo(int x) { ... }
  void bar(dynamic y) { ... }
}

class C extends B {
  void bar(String y) { ... }
}

[jmesserly]

Not sure if this is the right bug, but we've discussed covariant parameter types in overrides (and adding a check). I thought it was interesting that Eiffel supported this:
https://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science)#Contravariant_method_argument_type

I think we were considering some explicit annotation (on the base class method? Or on the derived class method?) to opt in.

[jmesserly]

merging into analyzer bug: dart-lang/sdk#25232

[jmesserly]

actually moved to: dart-lang/sdk#25578, the other bug turned out to not be strong mode specific