Introduces the new syntax for deferred loading

tests/lib/mirrors/deferred_mirrors_metadata_test.dart

 @A(const B())
 library main;
 
 @B()
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 
-import "dart:async";
 import "dart:math";
 
-@l import 'deferred_mirrors_metadata_lib.dart' as lib1;
-
-const l = const DeferredLibrary("lib1");
-
+import 'deferred_mirrors_metadata_lib.dart' deferred as lib1;
 
 class A {
   final B b;
   const A(this.b);
   String toString() => "A";
 }
 
 class B {
   const B();
   String toString() => "B";
 }
 
 class C {
   const C();
   String toString() => "C";
 }
 
 void main() {
   asyncStart();
-  l.load().then((_) {
+  lib1.loadLibrary().then((_) {
     Expect.equals("ABC", lib1.foo());
     asyncEnd();
   });
 }