JS: move expressions test to box tests

js/js.translator/testData/box/expression/cast/castToNullable.kt

@@ -0,0 +1,18 @@
+package foo
+
+interface A
+
+class AImpl : A {}
+
+fun test(x: Any?): A? = x as A?
+
+fun box(): String {
+    var a: A? = AImpl()
+    assertEquals(a, test(a), "a = AImpl()")
+    a = object : A {}
+    assertEquals(a, test(a), "a = object : A{}")
+    assertEquals(null, test(null), "test(null)")
+    failsClassCast("test(object{})") { test(object{}) }
+
+    return "OK"
+}