[js] Regression with null safety diagnostics

[RblSb]

In nightly builds after file resave this error occurs: (no compilation errors). From vshaxe codebase, doesn't happen on interp.
Main.hx:

using StringTools;

@:nullSafety
class Main {
	static function main() {}

	static function extractReturnType(hint:String):Void {
		// Null safety: Cannot unify String with { charCodeAt : Int -> Int }
		for (i => code in hint) {}
	}
}

-cp src
-main Main
--js bin/main.js

[RblSb]

Minimized example: StringTools.unsafeCodeAt("foo", 0).

This basically inlines return (cast s).charCodeAt(index):
https://github.com/HaxeFoundation/haxe/blob/development/std/StringTools.hx#L504

Not really a regression, happens since c4cc470

Unification errors here on TAbstract(Null, [TAbstract(Int, [])]) and TAbstract(Int, []) comparison (idk how is first type is maked here).
https://github.com/HaxeFoundation/haxe/blob/development/src/typing/nullSafety.ml#L233

Interestingly, this error does not happen if you copy unsafeCodeAt function to Main class and call it. But error will happen on file resave if you create separate class and call it like Tools.unsafeCodeAt("foo", 0), so it looks like server cache problem related to modules, and not a null safety cache thing.