ASTs may be created by various tools and built from nodes of
different files. An incorrectly constructed AST will likely
not print at all, but a (structurally) correct AST with bad
position information should still print structurally correct.

One heuristic used was to reset indentation when the filename
in the position information of nodes changed. However, this
can lead to wrong indentation for structurally correct ASTs.

Fix: Don't change the indentation in this case.

Related to issue 4300.

R=r
CC=golang-dev
https://golang.org/cl/6849066