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((_) {
+      lib2.foo("");
+    }).catchError(expectAsync((_) {
+      expect(true, true);
+      }), test: (e) => e is DeferredLoadException);
+  });
+}