Remove support for the DeferredLibrary annotation and remove tests of it

tests/lib/async/deferred/deferred_in_isolate_test.dart

 import 'dart:isolate';
 import 'dart:async';
 import 'package:unittest/unittest.dart';
 
-@a import 'deferred_in_isolate_lib.dart' as lib1;
-@b import 'deferred_library.dart' as lib2;
+import 'deferred_in_isolate_lib.dart' deferred as lib;
 
-const a = const DeferredLibrary("lib1");
-const b = const DeferredLibrary("NonExistingFile", uri: "wrong/");
-
-loadDeferred(ports) {
-  a.load().then((_) {
-    ports[0].send(lib1.f());
+loadDeferred(port) {
+  lib.loadLibrary().then((_) {
+    port.send(lib.f());
   });
-  b.load().then((b) {
-    ports[1].send("No error");
-    lib2.foo(20);
-  }).catchError((_) {
-    ports[1].send("Error caught");
-  }, test: (e) => e is DeferredLoadException);
 }
 
 main() {
   test("Deferred loading in isolate", () {
-    List<ReceivePort> ports = new List.generate(2, (_) => new ReceivePort());
-    ports[0].first.then(expectAsync((msg) {
+    ReceivePort port = new ReceivePort();
+    port.first.then(expectAsync((msg) {
        expect(msg, equals("hi"));
     }));
-    ports[1].first.then(expectAsync((msg) {
-      expect(msg, equals("Error caught"));
-    }));
-    Isolate.spawn(loadDeferred, ports.map((p) => p.sendPort).toList());
+    Isolate.spawn(loadDeferred, port.sendPort);
   });
 }