Custom context menu example (#1463)

* flutter create

* Copied in from 'contextual-menu-examples' repo

* Works again with the latest updates to the branch

* Clean up anywhere example

* Clean up other examples

* Add to CI

* Updated with release version of context menus, and added platform switcher

* Generated files

* Home icon on your original platform

* Remove web_dashboard from ci, not sure why that change was there...

* Add context_menu to beta channel, but commented out, for the future

* +x permissions on the master script, which I may have accidentally changed before?

* Actual bash comment

* dart format

* Natural default platform'

* A working test, for the email page

* Import order fix

* Test some pages

* More tests

* dart format

experimental/context_menus/windows/runner/flutter_window.h

@@ -0,0 +1,33 @@
+#ifndef RUNNER_FLUTTER_WINDOW_H_
+#define RUNNER_FLUTTER_WINDOW_H_
+
+#include <flutter/dart_project.h>
+#include <flutter/flutter_view_controller.h>
+
+#include <memory>
+
+#include "win32_window.h"
+
+// A window that does nothing but host a Flutter view.
+class FlutterWindow : public Win32Window {
+ public:
+  // Creates a new FlutterWindow hosting a Flutter view running |project|.
+  explicit FlutterWindow(const flutter::DartProject& project);
+  virtual ~FlutterWindow();
+
+ protected:
+  // Win32Window:
+  bool OnCreate() override;
+  void OnDestroy() override;
+  LRESULT MessageHandler(HWND window, UINT const message, WPARAM const wparam,
+                         LPARAM const lparam) noexcept override;
+
+ private:
+  // The project to run.
+  flutter::DartProject project_;
+
+  // The Flutter instance hosted by this window.
+  std::unique_ptr<flutter::FlutterViewController> flutter_controller_;
+};
+
+#endif  // RUNNER_FLUTTER_WINDOW_H_