the plugins.qmltypefile is not deployed to the correct directory, so Creator integration is broken.

Something like

diff --git a/Source/WebKit/qt/declarative/public.pri b/Source/WebKit/qt/declarative/public.pri
index 5fc5985..8cc9219 100644
— a/Source/WebKit/qt/declarative/public.pri
+++ b/Source/WebKit/qt/declarative/public.pri
@@ -49,6 +49,7 @@ target.path = $${importPath}/$${TARGET.module_name}

qmldir.files += $$PWD/qmldir
+qmldir.files += $$PWD/plugins.qmltypes
qmldir.path += $${importPath}/$${TARGET.module_name}

INSTALLS += target qmldir

fixes it.
Such a fix cannot break any app (not used at runtime), but fixes qmleditor/designer in creator, which is important to give a good developer experience, so there should be no problem in adding it to release.