[JS BE] add tests for es6-classes

js/js.translator/testData/box/es6classes/inheritedFromExternalClass.kt

@@ -0,0 +1,17 @@
+//IGNORE_BACKEND: JS, JS_IR
+
+open external class E(x: Int, y: Int) {
+    val t: Int = definedExternally
+}
+
+open class A(i: Int, j: Int) : E(i, j)
+
+class B(val ok: String) : A(2, 3)
+
+fun box(): String {
+    val b = B("OK")
+
+    assertEquals(5, b.t)
+
+    return b.ok
+}