specify when a Future completes for IOSink's addStream

sdk/lib/io/io_sink.dart

Index: sdk/lib/io/io_sink.dart
diff --git a/sdk/lib/io/io_sink.dart b/sdk/lib/io/io_sink.dart
index 89cd586cafbb14575bdc5b4a19f042526d2886ca..2d6719d6a8f43140ee3e916fd1b5c583ea9874be 100644
--- a/sdk/lib/io/io_sink.dart
+++ b/sdk/lib/io/io_sink.dart
@@ -97,6 +97,9 @@ abstract class IOSink implements StreamSink<List<int>>, StringSink {
 
   /**
    * Adds all elements of the given [stream] to `this`.
+   *
+   * Returns a [Future] that completes when
+   * all elements of the given [stream] are added to `this`.
    */
   Future addStream(Stream<List<int>> stream);