[test] Fix analyzer warning in the recently added test

Change-Id: I808e1b32a8c3b7bcd1da257116c06926d391b987
Reviewed-on: https://dart-review.googlesource.com/52862
Reviewed-by: Mike Fairhurst <mfairhurst@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>

runtime/tests/vm/dart/error_messages_in_null_checks_test.dart

@@ -13,7 +13,7 @@ class A {
     Expect.fail('A.foo should not be reachable');
   }
 
-  int get bar {
+  dynamic get bar {
     Expect.fail('A.bar should not be reachable');
   }