Use a `confuse` function.

tests/language/mixin_proto_test.dart

Index: tests/language/mixin_proto_test.dart
diff --git a/tests/language/mixin_proto_test.dart b/tests/language/mixin_proto_test.dart
index e195d904b4c21e22f1865f07f416cdc2acd7a822..e3967725ee0171c46c53daf2526934ffcf4e210b 100644
--- a/tests/language/mixin_proto_test.dart
+++ b/tests/language/mixin_proto_test.dart
@@ -26,11 +26,15 @@ class C extends A with B {
   bar() => 499;
 }
 
+@NoInline()
+@AssumeDynamic()
+confuse(x) => x;
+
 main() {
   var all = [new A(), new B(), new C()];
-  Expect.equals(44, all[0].foo());
-  Expect.equals(22, all[0].bar());
-  Expect.equals(42, all[1].foo());
-  Expect.equals(42, all[2].foo());
-  Expect.equals(499, all[2].bar());
+  Expect.equals(44, confuse(all[0]).foo());

[sra1, 2017/02/24 17:30:08]

FYI

You could also write  confuse(new A())

If you want an inferred type that is not dynamic, but s specific union, you can
use something like

confuseABC(x) (
  var a = confuse(x);
  if (a is A) return a;
  if (a is B) return a;
  if (a is C) return a;
  throw 123;
}

confuseABC(new A())  - dynamic A with static type {A,B,C}.

[floitsch, 2017/02/27 14:42:24]

Acknowledged. I don't think that's necessary here.
I guess that confusing the index would also be ok.
`all[confuse(0)]`

+  Expect.equals(22, confuse(all[0]).bar());
+  Expect.equals(42, confuse(all[1]).foo());
+  Expect.equals(42, confuse(all[2]).foo());
+  Expect.equals(499, confuse(all[2]).bar());
 }