[flutter_tools] fix language_version_test and enable shuffle (#142009)

Part of https://github.com/flutter/flutter/issues/85160

packages/flutter_tools/test/general.shard/dart/language_version_test.dart

@@ -2,13 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// TODO(gspencergoog): Remove this tag once this test's state leaks/test
-// dependencies have been fixed.
-// https://github.com/flutter/flutter/issues/85160
-// Fails with "flutter test --test-randomize-ordering-seed=456"
-@Tags(<String>['no-shuffle'])
-library;
-
 import 'package:file/memory.dart';
 import 'package:flutter_tools/src/base/file_system.dart';
 import 'package:flutter_tools/src/dart/language_version.dart';
@@ -252,6 +245,7 @@ part of 'foo.dart';
 
   testWithoutContext('does not detect language version after library declaration', () {
     final FileSystem fileSystem = MemoryFileSystem.test();
+    setUpLanguageVersion(fileSystem);
     final File file = fileSystem.file('example.dart')
       ..writeAsStringSync('''
 // Some license