Remove the core library stubbing infrastructure.

pkg/stub_core_library/lib/src/utils.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.
-
-library stub_core_libraries.utils;
-
-/// Returns a sentence fragment listing the elements of [iter].
-///
-/// This converts each element of [iter] to a string and separates them with
-/// commas and/or "and" where appropriate.
-String toSentence(Iterable iter) {
-  if (iter.length == 1) return iter.first.toString();
-  return iter.take(iter.length - 1).join(", ") + " and ${iter.last}";
-}
-
-/// Returns [name] if [number] is 1, or the plural of [name] otherwise.
-///
-/// By default, this just adds "s" to the end of [name] to get the plural. If
-/// [plural] is passed, that's used instead.
-String pluralize(String name, int number, {String plural}) {
-  if (number == 1) return name;
-  if (plural != null) return plural;
-  return '${name}s';
-}