Teach dart_package to understand the packages/ subdirectory

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.
 
 # This target will be built if no target is specified when invoking ninja.
 group("default") {
   testonly = true
 
   deps = [
     "//crypto:crypto_unittests",
     "//examples",
     "//mojo",
     "//mojom",
     "//services",
     "//shell",
     "//ui/gfx:gfx_unittests",
     "//ui/events:events_unittests",
-    "//third_party/dart-packages/crypto",
   ]
 
   if (!is_android && !is_win) {
     deps += [ "//tools/xdisplaycheck" ]
   }
 
   if (!is_asan) {
     deps += [ "//sky" ]
   }
 }
 
 # Deprecated name for the default build target.
 group("root") {
   testonly = true
   deps = [
     ":default",
   ]
 }