If you teleport to the sentinel island teleporter shrine outside (someone else mentioned a door to get outside, probably leading to the same spot, but I don't recall using it), you can jump up the nearby rocks to the area above and run over far enough there are no overhanging trees, and morph into a dragon. IIRC you can also run down the path there and either get to a spot on the edge to change, or jump off and morph before you get too far into the poison gas.
