Fix casts in ReadableStream tests for Scala 2.12.

Author: ptrdom

tests-shared/src/main/scala/org/scalajs/dom/tests/shared/BrowserTests.scala

@@ -12,6 +12,7 @@ import org.scalajs.dom.tests.shared.AsyncTesting.async
 import scala.concurrent.Future
 import scala.scalajs.js
 import scala.scalajs.js.Thenable.Implicits._
+import scala.scalajs.js.|
 
 trait BrowserTests {
 
@@ -26,11 +27,11 @@ trait BrowserTests {
 
  val stream = ReadableStream[Tuna](
  new ReadableStreamUnderlyingSource[Tuna] {
- start = { (controller: ReadableStreamController[Tuna]) =>
+ start = js.defined({ (controller: ReadableStreamController[Tuna]) =>
  controller.enqueue(Tuna("blue"))
  controller.enqueue(Tuna("red"))
  controller.close()
- }: js.Function1[ReadableStreamController[Tuna], Unit]
+ }): js.UndefOr[js.Function1[ReadableStreamController[Tuna], Unit | js.Promise[Unit]]]
  }
  )