[MC-77418] Crash when opening Minecraft 1.8.2-pre6

When opening Minecraft 1.8.2-pre6, it crashes immediately, and nothing I can think of can fix it. The chat log just says

Exception in thread "main" java.lang.NoClassDefFoundError: net/minecraft/client/main/Main
Caused by: java.lang.ClassNotFoundException: net.minecraft.client.main.Main
	at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:247)

~~Ad also the monospacing doesn't work.~~