[typer] bring back "is" deprecation warning under haxe-next

also activate haxe-next on unit tests

src/context/common.ml

@@ -453,6 +453,8 @@ let define_value com k v =
 let convert_define k =
 	String.concat "_" (ExtString.String.nsplit k "-")
 
+let is_next com = defined com HaxeNext
+
 let external_defined ctx k =
 	Define.raw_defined ctx.defines (convert_define k)
 

src/context/display/deprecationCheck.ml

@@ -133,6 +133,5 @@ let run_on_field ?(force=false) com cf = if_enabled ~force com (fun() -> run_on_
 let run ?(force=false) com = if_enabled ~force com (fun() -> run com)
 
 let check_is com name meta p =
-	()
-	(* if name = "is" && not (Meta.has Meta.Deprecated meta) then
-		warn_deprecation com "Using \"is\" as an identifier is deprecated" p *)
+	if is_next com && name = "is" && not (Meta.has Meta.Deprecated meta) then
+		warn_deprecation com "Using \"is\" as an identifier is deprecated" p

tests/unit/compile-each.hxml

@@ -10,4 +10,5 @@
 -lib utest
 -D analyzer-optimize
 -D analyzer-user-var-fusion
--D message-reporting=pretty
+-D message-reporting=pretty
+-D haxe-next