Add mDNS discovery to the Fletch compiler

pkg/fletchc/lib/src/verbs/x_discover_verb.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.
+
+library fletchc.verbs.x_discover_verb;
+
+import 'infrastructure.dart';
+
+import 'documentation.dart' show
+    discoverDocumentation;
+
+import '../driver/developer.dart' show
+    discoverDevices;
+
+const Action discoverAction =
+    const Action(discover, discoverDocumentation);
+
+Future<int> discover(AnalyzedSentence sentence, VerbContext context) async {
+  await discoverDevices();
+  return 1;
+}