We should make either:

  [Global]
  interface G { };

  interface A { };
  [Exposed=G] G implements A;

or:

  [Global]
  interface G { };

  [Exposed=G]
  interface A { };
  G implements A;

do the same as:

  [Global]
  interface G { };

  [Exposed=G]
  partial interface G { ... members of A... };