Avoid a crash when no raw

[ef4]

You can make the compiler crash by trying to compile this file:

argv[2..-1].join()

like this:

/bin/coffee --js --input sample.coffee --source-map-file sample.map

This change defends against the missing expression.raw, since I'm inferring from the surrounding code that raw is not always present on every node.

[michaelficarra]

We should fix the accidentally missing raw field.