Revert "Make Zone API strong mode clean."

tests/lib/async/zone_register_callback_unary_test.dart

Index: tests/lib/async/zone_register_callback_unary_test.dart
diff --git a/tests/lib/async/zone_register_callback_unary_test.dart b/tests/lib/async/zone_register_callback_unary_test.dart
index aa8f2b01240a352598a91749c2f5ee62fd06b986..67697fe0da76a828d7b5ce5b061a2f84bbbc3b6e 100644
--- a/tests/lib/async/zone_register_callback_unary_test.dart
+++ b/tests/lib/async/zone_register_callback_unary_test.dart
@@ -15,7 +15,7 @@ main() {
   Expect.identical(Zone.ROOT, Zone.current);
   Zone forked = Zone.current.fork(specification: new ZoneSpecification(
       registerUnaryCallback:
-          <R>(Zone self, ZoneDelegate parent, Zone origin, R f(arg)) {
+          (Zone self, ZoneDelegate parent, Zone origin, f(arg)) {
     // The zone is still the same as when origin.run was invoked, which
     // is the root zone. (The origin zone hasn't been set yet).
     Expect.identical(Zone.current, Zone.ROOT);