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/tool/spec/codegen_dart_protocol.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:analyzer_utilities/tools.dart';
@@ -1120,6 +1122,8 @@ class CodegenProtocolVisitor extends DartCodegenVisitor with CodeGenerator {
   void visitApi() {
     outputHeader(year: '2017');
     writeln();
+    writeln('// @dart = 2.9');
+    writeln();
     emitImports();
     emitClasses(getClassesToEmit());
   }