Upload/Download dart_snapshotter

mojo/dart/BUILD.gn

 # Copyright 2014 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("//mojo/public/mojo.gni")
+
 group("dart") {
   deps = [
     "//mojo/dart/apptest",
-    "//mojo/dart/dart_snapshotter($host_toolchain)",
     "//mojo/dart/http_load_test",
     "//mojo/dart/mojom",
     "//mojo/dart/mojo_services",
     "//mojo/dart/observatory_test",
   ]
+  if (!mojo_use_prebuilt_dart_snapshotter) {
+    deps += [ "//mojo/dart/dart_snapshotter($host_toolchain)" ]
+  }
 }