Move blackBoxFile() testData to box/ directory

Delete all test methods (and empty test classes), since they'll be
auto-generated

compiler/testData/codegen/box/classes/kt1918.kt

@@ -0,0 +1,20 @@
+class Bar {
+}
+
+trait Foo {
+    fun xyzzy(x: Any?): String
+}
+
+fun buildFoo(bar: Bar.() -> Unit): Foo {
+    return object : Foo {
+        override fun xyzzy(x: Any?): String {
+           (x as? Bar)?.bar()
+            return "OK"
+        }
+    }
+}
+
+fun box(): String {
+    val foo = buildFoo({})
+    return foo.xyzzy(Bar())
+}