Fixes NPE in Devirtualizer.getStaticImpl in incremental superdevmode.

In order of the Devirtualizer to work all the types that are
devirtualized need to be fully "loaded" by UnifyAst.

UnifyAst was fully flowing into JSOs but not into j.l.String which
is also Devirtualized.

Change-Id: I54cd82d90cd85ab69f03bb34635a6d767b321832
Fix NPE in GenerateJavaScriptAST in incremental superdevmode.

Compile time constant propagacion might need trace expressions that
are marked as constanst by JDT all the way into the fields providing
the values, even if those fields are not directly reachable from
any stale type.

This patch makes sure reachable static initializer are traversed.

Change-Id: I89ea096c92334d43373c04b6ab242844e544a3f5
Allow modifying class names from header rows in StackPanel.

Change-Id: I0096e23a3199d7270dfb08d3c4895a1c3a9dab61
Review-Link: https://gwt-review.googlesource.com/#/c/9516/