[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