Correction: add reflexive case to "derive a future type", first item

Author: eernstg

specification/dartLangSpec.tex

@@ -11727,7 +11727,7 @@ \subsection{Function Expressions}
   %% TODO(eernst): Come mixin classes and extension types: add them.
   If $T$ is a type which is introduced by
   a class, mixin, or enum declaration,
-  and if a direct or indirect superinterface
+  and if $T$ or a direct or indirect superinterface
   (\ref{interfaceSuperinterfaces})
   of $T$ is \code{Future<$U$>} for some $U$,
   then $T$ derives the future type \code{Future<$U$>}.