Remove support for the DeferredLibrary annotation and remove tests of it

tests/lib/async/deferred/deferred_fail_to_load_test.dart

-import 'dart:isolate';
-import 'dart:async';
-import 'dart:html';
-import 'package:unittest/unittest.dart';
-
-@a import 'deferred_in_isolate_lib.dart' as lib1;
-@b import 'deferred_library.dart' as lib2;
-
-const a = const DeferredLibrary("lib1");
-const b = const DeferredLibrary("NonExistingFile", uri: "wrong/wrong.js");
-
-main() {
-  test("Deferred loading failing to load", () {
-    a.load().then(expectAsync((_) {
-      expect(lib1.f(), equals("hi"));
-    }));
-    b.load().then((_) {
-      expect(false, true);
-      lib2.foo("");
-    }).catchError(expectAsync((_) {
-      expect(true, true);
-      }), test: (e) => e is DeferredLoadException);
-  });
-}