GN: Build the SDK with app snapshots instead of script snapshots.

utils/dartdoc/BUILD.gn

 # Copyright (c) 2016, the Dart 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 file.
 
 import("../invoke_dart.gni")
 
 application_snapshot("dartdoc") {
   main_dart = "../../third_party/pkg/dartdoc/bin/dartdoc.dart"
+  training_args = [ "--help" ]
   inputs = exec_script("../../tools/list_dart_files.py",
                        [rebase_path("../../third_party/pkg/dartdoc")],
                        "list lines")
 }