Change framebuffer.clear to use background color

samples/stm32f746g-discovery/turtle-graphics/turtle.dart

 // Copyright (c) 2016, the Dartino 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.
 //
 // This sample was inspired by the Dart Hilbert curve sample:
 // https://github.com/daftspaniel/dart-Hilbertcurve/blob/master/web/turtle.dart
 
 library turtle;
 
 import "dart:math";
 
 import 'package:stm32/lcd.dart';
 
 class Turtle {
   FrameBuffer _display;
   int _x;
   int _y;
   int _direction = 90;
   Color penColor = Color.red;
 
   /// Create a new Turtle, and set initial location.
   Turtle(this._display, {xPosition: 0, yPosition: 0}) {
+    _display.backgroundColor = Color.black;
     _display.clear();
     _x = xPosition;
     _y = yPosition;
   }
 
   /// Move forward [distance] pixels in the current direction with the pen down.
   forward(int distance) {
     double r = _direction * (PI / 180.0);
     int dx = (distance * sin(r)).toInt();
     int dy = (distance * cos(r)).toInt();
@@ skipping 19 matching lines @@
   turnRight(int degrees) {
     _turn(-degrees);
   }
 
   /// Move to [x, y] with the pen up.
   flyTo(int x, int y) {
     this._x = x;
     this._y = y;
   }
 }