Adjust definition of flatten (#2696)

Adjust definition of flatten to ensure that `flatten(X) == X` when `X` is a type variable which is unrelated to future types.

specification/dartLangSpec.tex

@@ -11268,16 +11268,12 @@ \subsection{Function Expressions}
 which is used below and in other sections, as follows:
 
 \begin{itemize}
-\item{} If $T$ is a type variable $X$ with bound $B$,
-  or $T$ is an intersection type
-  (\ref{intersectionTypes})
-  \code{$X$\,\,\&\,\,$B$},
-  then \DefEquals{\flatten{T}}{\flatten{B}}.
-
 \item{} If $T$ is \code{$S$?}\ for some $S$
   then \DefEquals{\flatten{T}}{\code{\flatten{S}?}}.
 
-\item{} If $T$ is \code{FutureOr<$S$>} then \DefEquals{\flatten{T}}{S}.
+\item{} If $T$ is \code{FutureOr<$S$>} bounded
+  (\ref{bindingActualsToFormals})
+  then \DefEquals{\flatten{T}}{S}.
 
 \item{} If $T$ implements \code{Future<$S$>}
   (\ref{interfaceSuperinterfaces})
@@ -11288,7 +11284,13 @@ \subsection{Function Expressions}
 
 \commentary{%
 This definition guarantees that for any type $T$,
-\code{$T <:$ FutureOr<$\flatten{T}$>}.%
+\code{$T <:$ FutureOr<$\flatten{T}$>}.
+Note that when $X$ is a type variable with bound $B$,
+it is possible that \flatten{X} is different from $X$:
+$B$ could, for some $S$, be \code{FutureOr<$S$>},
+or a type variable $Y$ with bound \code{FutureOr<$S$>},
+or a class $C$ that implements \code{Future<$S$>},
+or a type variable $X$ with bound $C$.%
 }
 
 \LMHash{}%