Make Zone API strong mode clean.

tests/lib/async/future_timeout_test.dart

Index: tests/lib/async/future_timeout_test.dart
diff --git a/tests/lib/async/future_timeout_test.dart b/tests/lib/async/future_timeout_test.dart
index 6020a815859bc28b9c381fc5ae3c0aad43300179..ac06377eb628e5f8dc4b9e5743564b1613ad705a 100644
--- a/tests/lib/async/future_timeout_test.dart
+++ b/tests/lib/async/future_timeout_test.dart
@@ -155,7 +155,8 @@ main() {
       return 42;
     };
     forked = Zone.current.fork(specification: new ZoneSpecification(
-        registerCallback: (Zone self, ZoneDelegate parent, Zone origin, f()) {
+        registerCallback:
+            <R>(Zone self, ZoneDelegate parent, Zone origin, R f()) {
       if (!identical(f, callback)) return f;
       registerCallDelta++; // Increment calls to register.
       expect(origin, forked);