Switch analysis_server to language 2.12, so null safety, but opt-out files.

This should allow doing partial migration, specifically protocol files,
which are imported by other libraries, but are a small library cycle
that does not import much outside of it.

Change-Id: I904c05d6d5b444ee9a9dbd1f7ada12aabdcc5165
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/193583
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

pkg/analysis_server/lib/src/plugin/notification_manager.dart

@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart = 2.9
+
 import 'dart:collection';
 
 import 'package:analysis_server/protocol/protocol_generated.dart' as server;

pkg/analysis_server/lib/src/plugin/plugin_locator.dart

@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart = 2.9
+
 import 'package:analyzer/file_system/file_system.dart';
 
 /// An object used to locate a plugin within a package.

pkg/analysis_server/lib/src/plugin/plugin_manager.dart

@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart = 2.9
+
 import 'dart:async';
 import 'dart:collection';
 import 'dart:convert';

pkg/analysis_server/lib/src/plugin/plugin_watcher.dart

@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart = 2.9
+
 import 'dart:convert';
 
 import 'package:analysis_server/src/plugin/plugin_locator.dart';

pkg/analysis_server/lib/src/plugin/request_converter.dart

@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart = 2.9
+
 import 'package:analysis_server/protocol/protocol_generated.dart' as server;
 import 'package:analyzer_plugin/protocol/protocol_generated.dart' as plugin;
 

pkg/analysis_server/lib/src/plugin/result_collector.dart

@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart = 2.9
+
 /// A function used to determine whether results should be collected for the
 /// file with the given [path].
 typedef ShouldCollectPredicate = bool Function(String path);

pkg/analysis_server/lib/src/plugin/result_converter.dart

@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart = 2.9
+
 import 'package:analysis_server/protocol/protocol_generated.dart' as server;
 import 'package:analysis_server/src/protocol/protocol_internal.dart' as server;
 import 'package:analyzer_plugin/protocol/protocol_common.dart';

pkg/analysis_server/lib/src/plugin/result_merger.dart

@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart = 2.9
+
 import 'dart:collection';
 
 import 'package:analysis_server/protocol/protocol_generated.dart';