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

mojo/public/dart/third_party/barback/lib/src/internal_asset.dart

+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/// This library exists so that pub can retain backwards-compatibility with
+/// versions of barback prior to 0.13.1.
+///
+/// In 0.13.1, `lib/src/internal_asset.dart` was moved to
+/// `lib/src/asset/internal_asset.dart. Pub needs to support all versions of
+/// barback back through 0.13.0, though. In order for this to work, it needs to
+/// be able to import "package:barback/src/internal_asset.dart" on all available
+/// barback versions, hence the existence of this library.
+library barback.internal_asset;
+
+export 'asset/internal_asset.dart';