Let deferred load work from worker isolates.

tests/lib/async/deferred/deferred_fail_to_load_test.dart

 import 'dart:isolate';
 import 'dart:async';
 import 'dart:html';
 import '../../../../pkg/unittest/lib/unittest.dart';
 
 @a import 'deferred_in_isolate_lib.dart' as lib1;
 @b import 'deferred_api_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);
   });
 }