Use explicit dynamic over void return types (flutter#99)

Some implementations are relying on passing values through the visit
methods. Make return types explicitly `dynamic` to match their implicit
types from before.

This highlights that extending the `Visitor` class and expecting to return
useful values is inherently risky. Not only will usages fall through to dynamic
and have no static errors for things like typos, there are also places where
the values will be silently discarded by the implementation in` Visitor`.