JS: move expressions test to box tests

js/js.translator/testData/box/dynamic/is.kt

@@ -0,0 +1,23 @@
+// FILE: is.kt
+package foo
+
+class A
+
+@native
+class B
+
+fun box(): String {
+    val a: dynamic = A()
+    testTrue { a is A }
+    testFalse { a is B }
+
+    val b: dynamic = B()
+    testTrue { b is B }
+    testFalse { b is A }
+
+    return "OK"
+}
+
+// FILE: is.js
+function B() {
+}