This implements FutureOr<T> in strong mode, otherwise it's ignored (treated as `dynamic`.
Also fixes strong mode's inference subtype function incorrectly treating `void` as a malformed type. This had the consequence of allowing `void` to be inferred as a type argument.
R=leafp@google.com, paulberry@google.com
Review-Url: https://codereview.chromium.org/2647833002 .