Move pub into sdk/lib/_internal.

utils/tests/pub/install/switch_source_test.dart

-// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS d.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 d.file.
-
-library pub_tests;
-
-import 'dart:io';
-
-import '../descriptor.dart' as d;
-import '../test_pub.dart';
-
-main() {
-  initConfig();
-  integration('re-installs a package if its source has changed', () {
-    d.dir('foo', [
-      d.libDir('foo', 'foo 0.0.1'),
-      d.libPubspec('foo', '0.0.1')
-    ]).create();
-
-    d.appDir([{"path": "../foo"}]).create();
-
-    schedulePub(args: ['install'],
-        output: new RegExp("Dependencies installed!\$"));
-
-    d.packagesDir({"foo": "0.0.1"}).validate();
-
-    servePackages([packageMap("foo", "1.2.3")]);
-
-    d.appDir([dependencyMap("foo", "any")]).create();
-
-    schedulePub(args: ['install'],
-        output: new RegExp("Dependencies installed!\$"));
-
-    d.packagesDir({"foo": "1.2.3"}).validate();
-  });
-}