We now wait until we see the completed prompt from a command before
proceeding. This seems to cut down on a spurious error I have seen
this afternoon.

Change-Id: Ic0a3481d8c265c3c3b4449ec7ac1c2752b85b0b6
Reviewed-on: https://go-review.googlesource.com/6691Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>