ignore `@proxy` in strong mode

tests/language_strong/deferred_no_such_method_test.dart

Index: tests/language_strong/deferred_no_such_method_test.dart
diff --git a/tests/language_strong/deferred_no_such_method_test.dart b/tests/language_strong/deferred_no_such_method_test.dart
index 6476a4f6e77b433546ebc7da63fdd75d1af35e90..5e041208c511e3a0976aacf028c54e6be4083fc5 100644
--- a/tests/language_strong/deferred_no_such_method_test.dart
+++ b/tests/language_strong/deferred_no_such_method_test.dart
@@ -10,7 +10,7 @@ import "deferred_no_such_method_lib.dart" deferred as lib;
 void main() {
   asyncStart();
   lib.loadLibrary().then((_) {
-    Expect.equals(42, new lib.C().nonExisting());
+    Expect.equals(42, (new lib.C() as dynamic).nonExisting());
     asyncEnd();
   });
 }