No change to $GOROOT/src, misc formatting.

Nice side-effect: almost 3% faster runs because it's much faster to compute
line number differences in the generated output than the incoming source.

Benchmark run, best of 5 runs, before and after:
BenchmarkPrint	     200	  12347587 ns/op
BenchmarkPrint	     200	  11999061 ns/op

Fixes #4504.

LGTM=adonovan
R=golang-codereviews, adonovan
CC=golang-codereviews
https://golang.org/cl/69260045