This is information taken from IsomPreview.java and ZoneRenderer.java in the isom folder in the zip file linked here: http://twitter.com/#!/jeb_/status/129568296600219648
It's likely that these files were released by accident.
Thanks to Fesh0r and LexManos.
Here is the information we can work out about the code of Minecraft pre-obfuscation:

Structure:
full class path = class name in MCP
    constructor = argument type(s)
    field(s) name(s) = (static) type
    method(s) name(s) = (static) type (argument type(s))

Data:
net.minecraft.Pos = ChunkCoordinates in MCP
    fields x, z = int

net.minecraft.world.level.Level = World in MCP
    constructor = DirectoryLevelStorage, String, LevelSettings
    method getSharedSpawnPos = Pos
    method getTile = Tile (int, int, int)
    method getBrightness = float (int, int, int)
    method getChunk = LevelChunk (int, int)
    fields depth, skyDarken = int
    field name = String

net.minecraft.world.level.LevelSettings = WorldSettings in MCP
    constructor = long, LevelSettings Enum, boolean, boolean

net.minecraft.world.level.storage.DirectoryLevelStorage = SaveHandler in MCP
    constructor = File, String, boolean

net.minecraft.world.level.chunk.LevelChunk = Chunk in MCP
    method isEmpty (getFalse in MCP) = boolean

net.minecraft.world.level.material.Material = Material in MCP
    field water = static Material

net.minecraft.world.level.tile.Tile = Block in MCP
    field tiles = static Tile[256]
    field material = Material
    method getTexture = int (int)