Rename internal Dart content handler unit testing library to _testing

mojo/public/dart/rules.gni

 # 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.
 
 # Rules to generate zipped applications for Dart.
 # Rules to generate dart-pkg and dart-pkg/packages.
 
 import("//build/module_args/mojo.gni")
 
 template("dartzip_package") {
@@ skipping 261 matching lines @@
     }
 
     script = rebase_path("mojo/public/tools/dart_pkg.py", ".", mojo_sdk_root)
     outputs = [
       stamp_file,
     ]
 
     inputs = [
                list_mojoms_script,
                script,
-             ] + rebase_path(invoker.sources) + rebase_path(sdk_ext_directory)
+             ] + rebase_path(invoker.sources)
 
     args = [
              "--package-name",
              package_name,
              "--gen-directory",
              rebase_path("$root_gen_dir/dart-gen"),
              "--pkg-directory",
              pkg_directory,
              "--package-root",
              package_root,
              "--stamp-file",
              rebase_path(stamp_file),
              "--package-sources",
            ] + rebase_path(invoker.sources) + [ "--mojom-sources" ] +
            rebase_path(mojom_sources, "", mojo_sdk_root) +
            [ "--sdk-ext-directories" ] + rebase_path(sdk_ext_directory)
   }
 }