Wire up Android back button in SkyShell

sky/examples/raw/hello_world.dart

 // 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.
 
 import "dart:math";
 import 'dart:sky';
 
 Picture draw(int a, int r, int g, int b) {
   double width = view.width;
   double height = view.height;
@@ skipping 12 matching lines @@
     view.scheduleFrame();
     return true;
   }
 
   if (event.type == "pointerup") {
     view.picture = draw(255, 0, 255, 0);
     view.scheduleFrame();
     return true;
   }
 
+  if (event.type == "back") {
+    print("Pressed back button.");
+    return true;
+  }
+
   return false;
 }
 
 void main() {
   print("Hello, world");
   view.picture = draw(255, 0, 255, 0);
   view.scheduleFrame();
 
   view.setEventCallback(handleEvent);
 }