Use PUB_HOSTED_SERVER to set server URL for pub tests.

sdk/lib/_internal/pub/lib/src/source/hosted.dart

 // Copyright (c) 2012, 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 pub.source.hosted;
 
 import 'dart:async';
 import 'dart:io' as io;
 import 'dart:json' as json;
 
@@ skipping 13 matching lines @@
 /// A package source that installs packages from a package hosting site that
 /// uses the same API as pub.dartlang.org.
 class HostedSource extends Source {
 
   final name = "hosted";
   final shouldCache = true;
 
   /// Gets the default URL for the package server for hosted dependencies.
   static String get defaultUrl {
     var url = io.Platform.environment["PUB_HOSTED_URL"];
-    if (url != null) return url;
+    if (url != null) {
+      log.fine("Got server $url from PUB_HOSTED_URL.");
+      return url;
+    }
+
     return "https://pub.dartlang.org";
   }
 
   /// Downloads a list of all versions of a package that are available from the
   /// site.
   Future<List<Version>> getVersions(String name, description) {
     var url = _makeUrl(description,
         (server, package) => "$server/api/packages/$package");
 
     log.io("Get versions from $url.");
@@ skipping 213 matching lines @@
   var name = description["name"];
   if (name is! String) {
     throw new FormatException("The 'name' key must have a string value.");
   }
 
   var url = description["url"];
   if (url == null) url = HostedSource.defaultUrl;
 
   return new Pair<String, String>(name, url);
 }