Pass lexer around so state is accessible and dependency is explicit.
In the process remove EOF -> '\n' conversion that has to be corrected
for when reporting errors.

Change-Id: If95564b70e7484dedc1f5348e585cd19acbc1243
Reviewed-on: https://go-review.googlesource.com/19819
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>