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.

engine/src/flutter/sky/packages/updater/BUILD.gn

@@ -0,0 +1,30 @@
+# 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)",
+  ]
+}

engine/src/flutter/sky/packages/updater/lib/main.dart

@@ -0,0 +1,14 @@
+// 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");
+}

engine/src/flutter/sky/packages/updater/pubspec.yaml

@@ -0,0 +1,9 @@
+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'

engine/src/flutter/sky/tools/run_sky_snapshot.py

@@ -0,0 +1,33 @@
+#!/usr/bin/env python
+# 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.
+
+import argparse
+import os
+import subprocess
+import sys
+
+SKY_TOOLS_DIR = os.path.dirname(os.path.abspath(__file__))
+SRC_ROOT = os.path.dirname(os.path.dirname(SKY_TOOLS_DIR))
+WORKBENCH = os.path.join(SRC_ROOT, 'sky', 'packages', 'workbench')
+
+def main():
+    parser = argparse.ArgumentParser(description='Builds a Dart snapshot file')
+    parser.add_argument('--main', type=str)
+    parser.add_argument('--compiler', type=str)
+    parser.add_argument('--package-root', type=str)
+    parser.add_argument('--snapshot', type=str)
+    args = parser.parse_args()
+
+    command = [
+        os.path.abspath(args.compiler),
+        '--package-root=%s' % os.path.abspath(args.package_root),
+        '--snapshot=%s' % os.path.abspath(args.snapshot),
+        os.path.abspath(args.main),
+    ]
+
+    subprocess.check_call(command, cwd=WORKBENCH)
+
+if __name__ == '__main__':
+    sys.exit(main())