There is no Code to warp to Johto as Johto is not available in Pokémon Emerald in any of it's coding. Only Hoenn is available in Pokémon Emerald. Only Pokémon Gold, Silver and Crystal, along with the Gold and Silver remakes, let the player access regions that aren't part of the main setting of the game.
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Mirage Island Game Shark CodeThere are no Gameshark Warp codes in Emerald to get to Mirage Island. There is a code but it will hardly work7572668C A25C844CUse this and maybe you can get there
4D2A5A81 4E69A107 Battle Frontier
you can only warp to the southern island in Emerald version if that's what you meant. You'd need a Gameshark for Emerald version.
Master code (it is required to be on): D8BAE4D9 4864DCE5 A86CDBA5 19BA49B3 Warp to Birth Island: 4A99A22B 58284D2D After you are done using the code, disable or remove the code to prevent you from looping the warp.