[lua] new dead code elimination and string tests

tests/unit/src/unit/issues/Issue6259.hx

@@ -0,0 +1,8 @@
+package unit.issues;
+class Issue6259 extends Test{
+	function test(){
+		function f(a) return a.b;
+		var res = f({b: (s) -> s.length})("6259");
+		eq(res,4);
+	}
+}