Initial steps to add source map support in dwc:
 - Switches to use a new style of code printer that can track source map locations.
 - Changed to use new code printer: all the indentation done by hand, passing source-map location marks as we are emitting code.
 - source-map file is generated, but URLs are relative to base-dir. Once we start putting all the pieces together we can revisit this.

Review URL: https://chromiumcodereview.appspot.com//12221099