[js] support exposing abstract impl classes (can be useful while porting js code to haxe)

typeload.ml

@@ -202,7 +202,7 @@ let make_module ctx mpath file tdecls loadp =
 				(match !decls with
 				| (TClassDecl c,_) :: _ ->
 					List.iter (fun m -> match m with
-						| ((Meta.Build | Meta.CoreApi | Meta.Allow | Meta.Access | Meta.Enum | Meta.Dce | Meta.Native),_,_) ->
+						| ((Meta.Build | Meta.CoreApi | Meta.Allow | Meta.Access | Meta.Enum | Meta.Dce | Meta.Native | Meta.Expose),_,_) ->
 							c.cl_meta <- m :: c.cl_meta;
 						| _ ->
 							()