popup: doesn't invalidate zIndex when anchor set while popup is hidden (close #1214)

close #1214
I'm not sure, but I thought that  assertion `message` should contain addition information about particular error, e.g.

~~~js
  › node -p "require('assert').fail(1, 0, 'one does not equal to zero')"

assert.js:92
  throw new assert.AssertionError({
        ^
AssertionError: one does not equal to zero
    at [eval]:1:19
~~~ 

From this point of view your messages in assertions look strange, sorry :)