Use the cached pubspec if possible for describing hosted packages.

utils/tests/pub/version_solver_test.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_update_test;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:unittest/unittest.dart';
@@ skipping 850 matching lines @@
   /// Keeps track of which package pubspecs have been requested. Ensures that a
   /// source is only hit once for a given package and that pub internally
   /// caches the results.
   final _requestedPubspecs = new Map<String, Set<Version>>();
 
   final String name;
   bool get shouldCache => true;
 
   MockSource(this.name);
 
+  Future<String> systemCacheDirectory(PackageId id) {
+    return new Future.value('${id.name}-${id.version}');
+  }
+
   Future<List<Version>> getVersions(String name, String description) {
     return new Future.sync(() {
       // Make sure the solver doesn't request the same thing twice.
       if (_requestedVersions.contains(description)) {
         throw new Exception('Version list for $description was already '
             'requested.');
       }
 
       _requestedVersions.add(description);
 
@@ skipping 84 matching lines @@
   };
 
   var match = new RegExp(r"(.*) from (.*)").firstMatch(description);
   if (match != null) {
     name = match[1];
     source = sourceNames[match[2]];
   }
 
   callback(isDev, name, source);
 }