Teach dart_package to understand the packages/ subdirectory

mojo/dart/apptest/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/public/dart/rules.gni")
 
 dart_package("apptest") {
+  uses_pub = true
   sources = [
     "apptest/apptest.dart",
   ]
   deps = [
     "//mojo/public/dart",
-    "//third_party/dart-packages/unittest",
   ]
 }