Add dep on dart_snapshotter_bin label in dartx rule

mojo/public/dart/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.
+
 import("../mojo_sdk.gni")
 import("rules.gni")
 
 dart_mojo_sdk_entrypoints = [
   "lib/application.dart",
   "lib/bindings.dart",
   "lib/core.dart",
 ]
 
 dart_mojo_sdk_sources = [
@@ skipping 47 matching lines @@
     "../interfaces",
   ]
 }
 
 group("dart") {
   deps = [
     ":dart_dartzip",
     ":dart_pkg",
   ]
 }