The current escape code panics when an action involves chain nodes.
Such nodes can be seen in the following situation:

{{ . | AAA.B }} - AAA being a registered function

The above expression is actually valid, because AAA could return a
map containing a B key. The tests in text/template explicitly
demonstrate this case.

Fix allIdents to cover also chain nodes.

While I was investigating this issue, I realized that the tests
introduced in similar CL 9621 were incorrect. Parse errors were
caught as expected, but for the wrong reason. Fixed them as well.
No changes in text/template code itself.

Fixes #10801

Change-Id: Ic9fe43b63669298ca52c3f499e2725dd2bb818a8
Reviewed-on: https://go-review.googlesource.com/10340Reviewed-by: Rob Pike <r@golang.org>