Minor refactor of how gas IDs are handled

code/game/objects/items/chrono_eraser.dm

@@ -240,9 +240,9 @@
 
 /obj/effect/chrono_field/return_air() //we always have nominal air and temperature
 	var/datum/gas_mixture/GM = new
-	GM.assert_gases("o2","n2")
-	GM.gases["o2"][MOLES] = MOLES_O2STANDARD
-	GM.gases["n2"][MOLES] = MOLES_N2STANDARD
+	GM.add_gases(/datum/gas/oxygen, /datum/gas/nitrogen)
+	GM.gases[/datum/gas/oxygen][MOLES] = MOLES_O2STANDARD
+	GM.gases[/datum/gas/nitrogen][MOLES] = MOLES_N2STANDARD
 	GM.temperature = T20C
 	return GM