Port touch-demo.sky to Dart and make it work in SkyShell

sky/shell/platform_view.h

Index: sky/shell/platform_view.h
diff --git a/sky/shell/platform_view.h b/sky/shell/platform_view.h
index 7f83cde08398090603ed6730d7df14b2360f222b..c6da2e3b6deae15059543de68fcf4bb3289f1de2 100644
--- a/sky/shell/platform_view.h
+++ b/sky/shell/platform_view.h
@@ -33,6 +33,9 @@ class PlatformView {
   explicit PlatformView(const Config& config);
   ~PlatformView();
 
+  void ConnectToViewportObserver(
+      mojo::InterfaceRequest<ViewportObserver> request);
+
   // Called from Java
   void Detach(JNIEnv* env, jobject obj);
   void SurfaceCreated(JNIEnv* env, jobject obj, jobject jsurface);