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

sky/services/viewport/viewport_observer.mojom

Index: sky/services/viewport/viewport_observer.mojom
diff --git a/sky/services/viewport/viewport_observer.mojom b/sky/services/viewport/viewport_observer.mojom
new file mode 100644
index 0000000000000000000000000000000000000000..5fc996d13bafb555f67f272ade3bef044618e569
--- /dev/null
+++ b/sky/services/viewport/viewport_observer.mojom
@@ -0,0 +1,12 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+module sky;
+
+import "sky/services/viewport/input_event.mojom";
+
+interface ViewportObserver {
+  OnViewportMetricsChanged(int32 width, int32 height, float device_pixel_ratio);

[eseidel, 2015/02/19 00:50:34]

unsigned?

+  OnInputEvent(InputEvent event);
+};