Switch list type to a generic tagged type. This is in preparation for browser-tagged types.
Switch list type to a generic tagged type. This is in preparation for
browser-tagged types.

chunmun/tree-builder-builder,dglazkov/tree-builder-builder,chunmun/tree-builder-builder,dglazkov/tree-builder-builder,chunmun/tree-builder-builder,dglazkov/tree-builder-builder