Make FileSystemEntity.watch return a broadcast stream.

sdk/lib/io/file_system_entity.dart

Index: sdk/lib/io/file_system_entity.dart
diff --git a/sdk/lib/io/file_system_entity.dart b/sdk/lib/io/file_system_entity.dart
index 80f1b39ee4ff86ee6f349787d6d512579e2e49a5..c42e3a75946884a820f9f59486826f864177408d 100644
--- a/sdk/lib/io/file_system_entity.dart
+++ b/sdk/lib/io/file_system_entity.dart
@@ -355,7 +355,8 @@ abstract class FileSystemEntity {
    * The system will start listening for events once the returned [Stream] is
    * being listened to, not when the call to [watch] is issued.
    *
-   * Note that the returned [Stream] is endless, unless:
+   * The returned value is an endless broadcast [Stream], that only stops when
+   * one of the following happends:
    *
    *   * The [Stream] is canceled, e.g. by calling `cancel` on the
    *      [StreamSubscription].