Fixed race in incorrect windows platform channel test. (flutter#37012)

Author: gaaclarke

shell/platform/windows/fixtures/main.dart

@@ -32,7 +32,7 @@ void hiPlatformChannels() {
       ui.PlatformDispatcher.instance
           .sendPlatformMessage('hi', reply, (ByteData? reply) {});
     });
-    callback(null);
+    callback(data);
   });
 }
 

shell/platform/windows/flutter_windows_engine_unittests.cc

@@ -295,12 +295,12 @@ TEST_F(FlutterWindowsEngineTest, PlatformMessageRoundTrip) {
   binary_messenger->Send(
       channel, reinterpret_cast<uint8_t*>(payload), 5,
       [&did_call_reply](const uint8_t* reply, size_t reply_size) {
-        EXPECT_EQ(reply_size, 3);
-        EXPECT_EQ(reply[0], static_cast<uint8_t>('b'));
+        EXPECT_EQ(reply_size, 5);
+        EXPECT_EQ(reply[0], static_cast<uint8_t>('h'));
         did_call_reply = true;
       });
   // Rely on timeout mechanism in CI.
-  while (!did_call_callback && !did_call_reply && !did_call_dart_reply) {
+  while (!did_call_callback || !did_call_reply || !did_call_dart_reply) {
     engine->task_runner()->ProcessTasks();
   }
 }