Remove support for the DeferredLibrary annotation and remove tests of it

tests/compiler/dart2js_extra/deferred/deferred_constant2_test.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // 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.
 
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 
 import 'dart:async';
 
-@lazy import 'deferred_class_library2.dart' as lib;
-
-const lazy = const DeferredLibrary('deferred_class_library2');
+import 'deferred_class_library2.dart' deferred as lib;
 
 main() {
   asyncStart();
-  lazy.load().then((_) {
+  lib.loadLibrary().then((_) {
     Expect.equals(499, lib.C1.value);
     asyncEnd();
   });
 }