covariant parameters should have a wider parameter type at runtime

Forked from https://github.com/dart-lang/sdk/issues/27353. When we have an `@checked` parameter, it is more user friendly to widen the parameter type in the tear off function. This allows it to be used in more places. It is sound because of the callee side parameter type check.
