Change flatten to include "bounded" in the first case (#2707)

Change flatten to include "bounded" in the first case. This is needed in order to ensure, e.g., with `X extends Future<A>?` that `flatten(X)` is `A?`.

This PR also adds a front matter comment about this change, and it changes the subheaders used in the front matter comment to use the format <month> <year>.

Author: eernstg

specification/dartLangSpec.tex

@@ -14,7 +14,7 @@
 \makeindex
 \title{Dart Programming Language Specification\\
 {6th edition draft}\\
-{\large Version 2.15-dev}}
+{\large Version 2.13-dev}}
 \author{}
 
 % For information about Location Markers (and in particular the
@@ -26,6 +26,19 @@
 %
 % Significant changes to the specification.
 %
+% Note that the version numbers used below (up to 2.15) were associated with
+% the currently released language and tools at the time of the spec change,
+% they were not aligned with the actual version of the language which is being
+% specified in that version of this document. This is highly misleading, so we
+% changed the subtitles to be a month rather than a version number. Similarly,
+% the 'Version' specified on the front page has been changed to indicate the
+% version of the language which will actually be specified by the next stable
+% release of this document.
+%
+% Dec 2022
+% - Change the definition of the type function 'flatten' to resolve soundness
+%   issue, cf. SDK issue #49396.
+%
 % 2.15
 % - Allow generic instantiation of expressions with a generic function type
 %   (until now, it was an error unless the expression denoted a declaration).
@@ -11268,11 +11281,11 @@ \subsection{Function Expressions}
 which is used below and in other sections, as follows:
 
 \begin{itemize}
-\item{} If $T$ is \code{$S$?}\ for some $S$
-  then \DefEquals{\flatten{T}}{\code{\flatten{S}?}}.
+\item{} If $T$ is \code{$S$?}\ bounded
+  (\ref{bindingActualsToFormals})
+  for some $S$ then \DefEquals{\flatten{T}}{\code{\flatten{S}?}}.
 
 \item{} If $T$ is \code{FutureOr<$S$>} bounded
-  (\ref{bindingActualsToFormals})
   then \DefEquals{\flatten{T}}{S}.
 
 \item{} If $T$ implements \code{Future<$S$>}