Stop running pub get at gclient sync time and fix build bugs

mojo/public/dart/mojom/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("../rules.gni")
 
 dart_pkg("mojom") {
   libs = [
     "bin/mojom.dart",
     "lib/src/generate.dart",
     "lib/src/command_runner.dart",
     "lib/src/commands/all.dart",
     "lib/src/commands/check.dart",
     "lib/src/commands/mojom_command.dart",
     "lib/src/commands/single.dart",
     "lib/src/commands/tree.dart",
     "lib/src/utils.dart",
     "test/generate_test.dart",
   ]
   sources = [
     "CHANGELOG.md",
     "README.md",
     "pubspec.yaml",
   ]
+  deps = [
+    "../third_party",
+  ]
 }