Add tests for the window controller on Windows (#41)

Author: Saviq

engine/src/flutter/shell/platform/windows/fixtures/main.dart

@@ -405,6 +405,11 @@ void testEngineId() {
   notifyEngineId(ui.PlatformDispatcher.instance.engineId);
 }
 
+@pragma('vm:entry-point')
+void testWindowController() {
+  signal();
+}
+
 @pragma('vm:entry-point')
 Future<void> sendSemanticsTreeInfo() async {
   // Wait until semantics are enabled.