[MCL-412] OS X: Exception in thread "main" java.lang.NoClassDefFoundError: net/minecraft/client/main/Main

Upgraded to 0.9 Dev Launcher after using 0.8.whateverTheLastVersionWas. Minecraft does not launch. Edited default profile to force it to use a version of Minecraft that I know had been downloaded by the previous version of Dev Launcher, and that shows a status of "Up to date" in the Version Selection area of the GUI. Still will not launch. Service status shows "Online, no problems" for both Multiplayer and Login. Attempting to run Minecraft results in...

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