Rules for source dump character escape changed in V8

[mraleph]

As a result unescaping code

https://github.com/mraleph/irhydra/blob/13a2cd5fe550941e9e0b7ad6f38db99e8e66ca27/lib/src/modes/v8/code_parser.dart#L119-L129

no longer works. It is not obvious to me how to fix this because I don't see a backwards compatible way to detect whether escaping was applied or not.