Without some hook like this, it's impossible to get a $ into the generate
command, which is necessary if you're trying to do some shell scripting
or regular expressions.

We could use backslash escaping but that's already tricky enough
because the strings are processed as Go strings. Using $ like this
means we need no more mechanism, just a predefined variable.

We may need to revisit this but I hope we can avoid new quoting rules.

Change-Id: Ieb478c8cc767a866765282472239ed3c1e5669a8
Reviewed-on: https://go-review.googlesource.com/8091Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Ian Lance Taylor <iant@golang.org>