JSO dispatch in JSNI was only sometimes blocked, fixed.

The enforcement of blocking JSO dispatch in JSNI depended on a 
datastructure that was not guarranteed to yet be fully populated.

Moved the enforcement to a later compiler stage.

Change-Id: I5867ef061a36095e1393e06e289d6cc2e1e1e692
Review-Link: https://gwt-review.googlesource.com/#/c/10230/