Remove support for the DeferredLibrary annotation and remove tests of it

tests/compiler/dart2js_extra/deferred/deferred_constant2_test.dart

Index: tests/compiler/dart2js_extra/deferred/deferred_constant2_test.dart
diff --git a/tests/compiler/dart2js_extra/deferred/deferred_constant2_test.dart b/tests/compiler/dart2js_extra/deferred/deferred_constant2_test.dart
index 21c4a7d1b2442521cd007df6ef08f736100e8056..6ea37523cd75c7fc337001068c4b76181767da54 100644
--- a/tests/compiler/dart2js_extra/deferred/deferred_constant2_test.dart
+++ b/tests/compiler/dart2js_extra/deferred/deferred_constant2_test.dart
@@ -7,13 +7,11 @@ 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();
   });