IO: Fix FileSystemEntity.get:parent comment.

It claimed that `dir.parent` returns `dir` if `dir` is a root directory.

However in reality it always returned a new directory object.

R=kustermann@google.com

Review-Url: https://codereview.chromium.org/2668653002 .

sdk/lib/io/file_system_entity.dart

@@ -653,8 +653,7 @@ abstract class FileSystemEntity {
   }
 
   /**
-   * The directory containing [this].  If [this] is a root
-   * directory, returns [this].
+   * The directory containing [this].
    */
   Directory get parent => new Directory(parentOf(path));