JS fix gradle example

libraries/tools/kotlin-gradle-plugin/src/test/resources/testProject/kotlin2JsProject/mainProject/src/main/kotlin/example/main.kt

@@ -7,12 +7,12 @@ import example.library.Counter
 fun main(args: Array<String>) {
     val el = document.createElement("div")
     el.appendChild(document.createTextNode("Hello!"))
-    document.body.appendChild(el)
+    document.body!!.appendChild(el)
 
     val counterDiv = document.createElement("div")
     val counterText = document.createTextNode("Counter!")
     counterDiv.appendChild(counterText)
-    document.body.appendChild(counterDiv)
+    document.body!!.appendChild(counterDiv)
 
     val counter = Counter(counterText)
     counter.start()