Replace C++ UpdateTask with an empty dart version, loaded as a snapshot.

This adds a new placeholder dart package updater in sky/packages. This is built
into a snapshot and compiled into the Sky engine binary using the same
mechanism as the Dart isolate.

I also added a SkyHeadless class, similar to SkyView, used for running Dart
code without a view.

packages/updater/BUILD.gn

+# Copyright 2015 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+action("updater") {
+  snapshot = "$target_gen_dir/updater_snapshot.bin"
+  main_dart = "lib/main.dart"
+
+  inputs = [
+    main_dart,
+  ]
+  outputs = [
+    snapshot,
+  ]
+
+  sky_snapshot_dir =
+      get_label_info("//sky/tools/sky_snapshot($host_toolchain)", "root_out_dir")
+  script = "//sky/tools/run_sky_snapshot.py"
+
+  args = [
+    "--compiler", rebase_path("$sky_snapshot_dir/sky_snapshot", root_build_dir),
+    "--package-root", rebase_path("packages", root_build_dir),
+    "--snapshot", rebase_path(snapshot, root_build_dir),
+    "--main", rebase_path(main_dart, root_build_dir),
+  ]
+
+  deps = [
+    "//sky/tools/sky_snapshot($host_toolchain)",
+  ]
+}

packages/updater/lib/main.dart

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+class UpdateTask {
+  UpdateTask() {}
+
+  String toString() => "UpdateTask()";
+}
+
+void main() {
+  var x = new UpdateTask();
+  print("Success: $x");
+}

packages/updater/pubspec.yaml

+name: sky_updater
+version: 0.0.1
+author: Chromium Authors <sky-dev@googlegroups.com>
+description: The autoupdater for flutter
+homepage: http://flutter.io
+dependencies:
+  mojo: ^0.0.21
+environment:
+  sdk: '>=1.12.0 <2.0.0'