Let deferred load work from worker isolates.

tests/lib/async/deferred/deferred_in_isolate_test.dart

+import 'dart:isolate';
+import 'dart:async';
+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/");
+
+loadDeferred(ports) {
+  a.load().then((_) {
+    ports[0].send(lib1.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) {
+       expect(msg, equals("hi"));
+    }));
+    ports[1].first.then(expectAsync((msg) {
+      expect(msg, equals("Error caught"));
+    }));
+    Isolate.spawn(loadDeferred, ports.map((p) => p.sendPort).toList());
+  });
+}