assert: manutally create stack trace message

v8 has changed how it generates it's messages, so as a fix the stack is
manually manipulated after to properly display the assert message text.

This change is absolute crap, and I don't like manipulating the stack trace string this way. But so far it's the only way I've figured out how to do it. Will look for a better way for a little longer, but go ahead and use this if you want.