Refactor Raspberry Pi GPIO and add GPIO mock

samples/raspberry_pi/basic/buzzer.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 small buzzer example illustrating both input and output GPIO pins.
 //
 // 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';
 
 main() {
   // GPIO pin constants.
   const int button = 16;
   const int speaker = 21;
 
-  // Initialize GPIO and configure the pins.
-  PiMemoryMappedGPIO gpio = new PiMemoryMappedGPIO();
+  // Initialize Raspberry Pi and configure the pins.
+  RaspberryPi pi = new RaspberryPi();
+  PiMemoryMappedGPIO gpio = pi.memoryMappedGPIO;
   gpio.setMode(button, Mode.input);
   gpio.setMode(speaker, Mode.output);
 
   // Map state of button to speaker in a continuous loop.
   while (true) {
     bool buttonState = gpio.getPin(button);
     gpio.setPin(speaker, buttonState);
   }
 }