This reverts commit cd0a5f08, which
unnecessarily restricts the use of AddParseTree.

Change-Id: I1155214a20ba08981d604404e79fff54874fd8e4
Reviewed-on: https://go-review.googlesource.com/83919
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>