Simplifies the way linkers add code at the beginning of the output.

Linkers used a complex process to keep the output JS and the
corresponding sourcemaps in sync by adding a synthetic artifact that
represented the editions of the source.

This patch simplifies the process by only letting the linker add code
the first line (line breaks are stripped from the module prefix) thus
making the sourcemap artifact be valid after edition.

Change-Id: If4e033ef92a95172f739ef7dbb88de49b2369c87
22 files changed