GN: Don't depend on dart_bootstrap if there is a usable prebuilt sdk

utils/pub/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")
+import("../application_snapshot.gni")
 
 application_snapshot("pub") {
   main_dart = "../../third_party/pkg/pub/bin/pub.dart"
   training_args = [ "--help" ]
   deps = [
     "../compiler:dart2js_files_stamp",
   ]
   inputs = [
     "$root_gen_dir/dart2js_files.stamp",
   ]
 }