Dart: Runs dartanalyzer for the dart_pkg build rule.

mojo/dart/testing/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_pkg("testing") {
-  sources = [
+  entrypoints = [
     "lib/async_helper.dart",
     "lib/expect.dart",
     "lib/validation_test_input_parser.dart",
+  ]
+  sources = [
     "pubspec.yaml",
   ]
 }