I decided to do some testing for a slime farm in a super flat world. I used the preset:
3;minecraft:bedrock,2*minecraft:stone,minecraft:bedrock;1;village

To prevent all spawning by default. I searched for a slime-chunk and dug out the top most bedrock layer. A slime spawned and identified this as a valid slime chunk.

Than I covered the stone with torches, effectively removing all air blocks. This should prevent slimes form spawning on the stone again.

But a slime spawnt anyway, even so no airblock was present at the whole generated world, at y-lvl 3

I created a world with the preset
3;minecraft:bedrock,2*minecraft:stone,minecraft:torch;1;village to further test this.

the attached images show the same world in 1.8 and 1.7.10 enviroment.
In 1.8 slimes do spawn all over the place, the same world, loaded in 1.7.10 version of the game had no Slime spawning at all, like it should be?
testing this with a rail on the top most layer gave the same result. as long as its not a full block, the spawn cycle seems to start anyway and is not restricted to air-blocks, like it used to be.

Making, for example, spawnproofing rails nearly impossible. The cycle starts on the track itself now and you can't place a full block ontop of the tracks without taking suffocation dmg. Covering the tracks with slabs would still allow slimes and baby zombies / pigmen to spawn.