Remove support for the DeferredLibrary annotation and remove tests of it

tests/compiler/dart2js/deferred_emit_type_checks_test.dart

Index: tests/compiler/dart2js/deferred_emit_type_checks_test.dart
diff --git a/tests/compiler/dart2js/deferred_emit_type_checks_test.dart b/tests/compiler/dart2js/deferred_emit_type_checks_test.dart
index 68a5d9b8e4805d1bc362cfcf2f1d88f2493dd4fb..bb9449a346b4f1e6d994b25cb6929bd8ad056793 100644
--- a/tests/compiler/dart2js/deferred_emit_type_checks_test.dart
+++ b/tests/compiler/dart2js/deferred_emit_type_checks_test.dart
@@ -63,14 +63,10 @@ void main() {
 // it with a type argument, and testing for the type. The extra support should
 // go to the deferred hunk.
 const Map MEMORY_SOURCE_FILES = const {"main.dart": """
-import "dart:async";
-
-@def import 'lib.dart' as lib show f, A, instance;
-
-const def = const DeferredLibrary("deferred");
+import 'lib.dart' deferred as lib show f, A, instance;
 
 void main() {
-  def.load().then((_) {
+  lib.loadLibrary().then((_) {
     print(lib.f(lib.instance));
   });
 }