Use PUB_HOSTED_SERVER to set server URL for pub tests.

sdk/lib/_internal/pub/test/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', () {
+    servePackages([packageMap("foo", "1.2.3")]);
+
     d.dir('foo', [
       d.libDir('foo', 'foo 0.0.1'),
       d.libPubspec('foo', '0.0.1')
     ]).create();
 
     d.appDir([{"path": "../foo"}]).create();
 
     pubInstall();
 
     d.packagesDir({"foo": "0.0.1"}).validate();
-    servePackages([packageMap("foo", "1.2.3")]);
     d.appDir([dependencyMap("foo", "any")]).create();
 
     pubInstall();
 
     d.packagesDir({"foo": "1.2.3"}).validate();
   });
 }