[os][stream] Refine return types after dropping Node.js v8

app/current/src/main/scala/io/scalajs/nodejs/os/OS.scala

@@ -133,8 +133,7 @@ trait OS extends js.Object {
     * Returns the system uptime in seconds.
     * @example os.uptime()
     */
-  // TODO: Return type should be Int after dropping Node.js 8 (Windows returns decimal until Node.js 10))
-  def uptime(): Double = js.native
+  def uptime(): Int = js.native
 
   /**
     * The os.userInfo() method returns information about the currently effective user -- on POSIX platforms,

app/current/src/main/scala/io/scalajs/nodejs/stream/Stream.scala

@@ -267,13 +267,12 @@ sealed trait IWritable extends LegacyStream {
 
   def destroyed: Boolean = js.native
 
-  // TODO: Return type should be this.type if node.js v8 dropped
-  def end(chunk: String | Uint8Array, callback: js.Function1[Error, Any]): js.UndefOr[this.type] = js.native
-  def end(chunk: String | Uint8Array): js.UndefOr[this.type]                                     = js.native
+  def end(chunk: String | Uint8Array, callback: js.Function1[Error, Any]): this.type = js.native
+  def end(chunk: String | Uint8Array): js.UndefOr[this.type]                         = js.native
 
-  def end(chunk: String, encoding: String, callback: js.Function1[Error, Any]): js.UndefOr[this.type] = js.native
-  def end(chunk: String, encoding: String): js.UndefOr[this.type]                                     = js.native
-  def end(callback: js.Function1[Error, Any] = js.native): js.UndefOr[this.type]                      = js.native
+  def end(chunk: String, encoding: String, callback: js.Function1[Error, Any]): this.type = js.native
+  def end(chunk: String, encoding: String): this.type                                     = js.native
+  def end(callback: js.Function1[Error, Any] = js.native): this.type                      = js.native
 
   /**
     * Sets the default encoding for a writable stream.