Fixes an Oracle config property lookup bug.

Out of context cleanup/review of DynamicPropertyOracle introduced a
bug by confusing com.google.gwt.dev.cfg.ConfigurationProperty and
com.google.gwt.core.ext.ConfigurationProperty.

Fixed the bug, added a regression test and added a TODO to get rid
of this duplicate class name confusion.

Change-Id: I0f7f4770ee7103a8358163e6cd986be6a04bd679