Migration tool incremental opt out emits incorrect version markers

[leafpetersen]

Running the migration tool from flutter master.

I ran the tool, and chose to not migrate several files, letting the migration tool mark them as opted out. The tool produced (for certain files) the following diff:

 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 import 'package:flutter/foundation.dart';
+
+// @dart=2.9
+
 import 'package:flutter/material.dart';

Note that the language version marker is after the first directive, which is invalid and causes it to be ignored by the tooling.

cc @stereotype441 @srawlins