[CFE] Serialize language version

Change-Id: I6ea448df722a7e1e51c31f06d42bfbe743855a45
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/111725
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Vyacheslav Egorov <vegorov@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>

pkg/kernel/binary.md

@@ -139,7 +139,7 @@ type CanonicalName {
 
 type ComponentFile {
   UInt32 magic = 0x90ABCDEF;
-  UInt32 formatVersion = 26;
+  UInt32 formatVersion = 27;
   List<String> problemsAsJson; // Described in problems.md.
   Library[] libraries;
   UriSource sourceMap;