Add a small LED interface and demo app.

dart/pkg/lk/lib/leds.dart

Index: dart/pkg/lk/lib/leds.dart
diff --git a/dart/pkg/lk/lib/leds.dart b/dart/pkg/lk/lib/leds.dart
new file mode 100644
index 0000000000000000000000000000000000000000..cb605a56151f762fe840eb250cc3f7c1c6fa538a
--- /dev/null
+++ b/dart/pkg/lk/lib/leds.dart
@@ -0,0 +1,22 @@
+// 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 _toggleLED = ForeignLibrary.main.lookup('led_toggle');
+
+class LED {
+  final int _id;
+
+  const LED._internal(this._id);

[lukechurch, 2016/02/09 09:20:01]

Consider an explanatory comment as to the pattern that you're using here, e.g.
the use of privacy to stop people accidentally creating extra LEDs that have no
corresponding hardward

[herhut, 2016/02/09 10:10:15]

I think this is a very common pattern and we should not describe it in every
system library where it is used. 

[lukechurch, 2016/02/09 10:13:53]

I agree. We should describe it once, in the place where people look before
writing their first hardware wrapper library in Dartino. If this isn't expected
to be that place, we can leave it out.

+
+  setState(bool value) {

[Søren Gjesse, 2016/02/09 09:16:01]

Just make this a setter.

And please add a getter for state and a toggle method (see comments in previous
file).

[herhut, 2016/02/09 10:10:15]

Done.

+    _toggleLED.icall$2(_id, value ? 1 : 0);
+  }
+}
+
+const LED0 = const LED._internal(0);
+const LED1 = const LED._internal(1);
+const LED2 = const LED._internal(2);
+const LED3 = const LED._internal(3);