Since updating to snapshot 18w33a, hostile mobs are spawning in all light conditions.  I.E. Zombies and Skeletons in the open during the day, or inside a brightly lit base.

Additionally, slimes are spawning outside of slime chunks at surface level - not in a swamp biome.  Bats are spawning in the open, nowhere near a cave entrance.

(Screenshots depict Creeper that spawned in brightly lit hallway, Bats, and slime that spawned at surface level of Sunflower plains, and another shot of the same showing my "F3" data)