don't run the summary code if the upgrade fails (#6350)

packages/flutter_tools/lib/src/commands/upgrade.dart

@@ -47,11 +47,11 @@ class UpgradeCommand extends FlutterCommand {
       mapFunction: (String line) => matchesGitLine(line) ? null : line
     );
 
-    await buildUnlinkedForPackages(Cache.flutterRoot);
-
     if (code != 0)
       return code;
 
+    await buildUnlinkedForPackages(Cache.flutterRoot);
+
     // Check for and download any engine and pkg/ updates.
     // We run the 'flutter' shell script re-entrantly here
     // so that it will download the updated Dart and so forth