Refactor Raspberry Pi GPIO and add GPIO mock

samples/raspberry_pi/basic/door-bell.dart

 // Copyright (c) 2015, the Fletch project authors. Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE.md file.
 //
 // A door bell example. Illustrates watching GPIO pin state with events.
 //
 // For breadboard layout and connections to the Pi, see:
 // https://storage.googleapis.com/fletch-archive/images/buzzer-schematic.png
 
 import 'package:gpio/gpio.dart';
+import 'package:raspberry_pi/raspberry_pi.dart';
 import 'package:os/os.dart';
 
 main() {
   // GPIO pin constants.
   const int button = 16;
   const int speaker = 21;
 
-  // Initialize GPIO and speaker pin.
-  SysfsGPIO gpio = new SysfsGPIO();
+  // Initialize Raspberry Pi and configure the pins.
+  RaspberryPi pi = new RaspberryPi();
+  SysfsGPIO gpio = pi.sysfsGPIO;
+
+  // Initialize pins.
   gpio.exportPin(speaker);
   gpio.setMode(speaker, Mode.output);
-
-  // Initialize button pin. Enable a button down trigger.
   gpio.exportPin(button);
   gpio.setMode(button, Mode.input);
   gpio.setTrigger(button, Trigger.both);
 
   // Continuously monitor button.
   while (true) {
     // Wait for button press.
     gpio.waitFor(button, true, -1);
 
     // Sound bell
     for (var i = 1; i <= 3; i++) {
       gpio.setPin(speaker, true);
       sleep(100);
       gpio.setPin(speaker, false);
       sleep(500);
     }
   }
 }