Source map deviates from V3 specs, ';' should be separator, not terminator

Hi,

First of all thank you for the effort invested in the closure compiler!

The closure compiler version 1.8.1 produces a sourcemap whose mappings part ends with ;
This isn't correct, the Rev.3 specs say ; is a separator, not a terminator.
This results in an error when separating groups in Python with .split (';') as I am trying to do in the [Transcrypt](http://www.transcrypt.org) Python to JS compiler, in order to support multi-level sourcemaps. Of course it can be circumvented, but it would be better to follow syntax specs exactly in my view.

Also starting the sourcemap with 'A' rather than 'AAAA' creates a needless special case.
This is within specs, but maybe it should be avoided, since parsing something regular can be faster and the benefits of saving three chars are negligeable. Of course if many segments are 'shortened' this changes, but that isn't the case here.

Kind regards
Jacques