Replace Node#first_line_source with Node#source_code

While slightly more expensive to compute, the `source_code` method is
conceptually clearer and aesthetically better-named.

Replace all implementations using `first_line_source` to use
`source_code` instead.

Change-Id: I0f564161040d14823d5b117080712b6229c3386c
Reviewed-on: http://gerrit.causes.com/45302
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>