Fix the old-hijack test.

We were testing for `is ZoneUnaryCallback` on a function returned by
expectAsync(). It turns out the expectAsync() wrappers make all
arguments optional, so `is ZoneUnaryCallback` returns true even if the
original function took two arguments. So instead, we test for
`is! ZoneBinaryCallback`.

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1648723002 .
diff --git a/lib/src/request.dart b/lib/src/request.dart
index b4d0e03..e0aa8ea 100644
--- a/lib/src/request.dart
+++ b/lib/src/request.dart
@@ -246,7 +246,7 @@
       throw new StateError("This request can't be hijacked.");
     }
 
-    if (callback is ZoneUnaryCallback) {
+    if (callback is! ZoneBinaryCallback) {
       var oldCallback = callback;
       callback = (stream, sink) {
         oldCallback(new StreamChannel<List<int>>(stream, sink));