Add a small LED interface and demo app.

dart/pkg/lk/lib/leds.dart

+// Copyright (c) 2016, the SoD 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.
+
+import 'dart:dartino.ffi';
+
+final ForeignFunction _setGpio = ForeignLibrary.main.lookup('gpio_set');
+final ForeignFunction _getGpio = ForeignLibrary.main.lookup('gpio_get');
+final ForeignFunction _getLedGpio = ForeignLibrary.main.lookup('led_get_gpio');
+
+class LED {
+  final int _id;
+
+  const LED._internal(this._id);
+
+  int get _gpio {
+    int gpio = _getLedGpio.icall$1(_id);
+    if (gpio < 0) throw new ArgumentError();
+    return gpio;
+  }
+
+  set state(bool value) => _setGpio.vcall$2(_gpio, value ? 1 : 0);
+
+  bool get state => _getGpio.icall$1(_gpio) == 1 ? true : false;
+
+  toggle() => state = !state;
+}
+
+const LED0 = const LED._internal(0);
+const LED1 = const LED._internal(1);
+const LED2 = const LED._internal(2);
+const LED3 = const LED._internal(3);