- checking for the correct closing token leads to slightly better
  behavior for some randomly bogus programs
- removed `switch` in favor of an `if` statement

Follow-up on https://go-review.googlesource.com/c/go/+/71250.

Change-Id: I47f6c47b43baf790907f55ed97a947661687a9db
Reviewed-on: https://go-review.googlesource.com/71252Reviewed-by: Matthew Dempsky <mdempsky@google.com>