JS: move expressions test to box tests

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

@@ -0,0 +1,16 @@
+package foo
+
+class A
+class B
+
+fun box(): String {
+    val a: Any? = A()
+    val nil: Any? = null
+    val b: Any? = B()
+
+    assertEquals(a, a as? A?, "a")
+    assertEquals(null, nil as? A?, "nil")
+    assertEquals(null, b as? A?, "b")
+
+    return "OK"
+}