Remove support for the DeferredLibrary annotation and remove tests of it

tests/compiler/dart2js_extra/deferred/deferred_function_test.dart

 // Copyright (c) 2013, 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.
 
 // Test that loading of a library (with top-level functions only) can
 // be deferred.
 
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 
-import 'dart:async';
-
-@lazy import 'deferred_function_library.dart' as lib;
-
-const lazy = const DeferredLibrary('deferred_function_library');
+import 'deferred_function_library.dart' deferred as lib;
 
 isError(e) => e is Error;
 
 readFoo() {
   return lib.foo;
 }
 
 main() {
   Expect.throws(() { lib.foo('a'); }, isError);
   Expect.throws(readFoo, isError);
   int counter = 0;
   asyncStart();
-  lazy.load().then((_) {
+  lib.loadLibrary().then((_) {
     Expect.equals(1, ++counter);
     print('lazy was loaded');
     Expect.equals(42, lib.foo('b'));
     Expect.isNotNull(readFoo());
     asyncEnd();
   });
   Expect.equals(0, counter);
   asyncStart();
-  lazy.load().then((_) {
+  lib.loadLibrary().then((_) {
     Expect.equals(2, ++counter);
     print('lazy was loaded');
     Expect.equals(42, lib.foo('b'));
     Expect.isNotNull(readFoo());
     asyncEnd();
   });
   Expect.equals(0, counter);
   Expect.throws(() { lib.foo('a'); }, isError);
   Expect.throws(readFoo, isError);
 }