Add a "cache add" command to download packages to the cache.

sdk/lib/_internal/pub/test/descriptor.dart

 // Copyright (c) 2013, 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.
 
 /// Pub-specific scheduled_test descriptors.
 library descriptor;
 
 import 'package:oauth2/oauth2.dart' as oauth2;
 import 'package:scheduled_test/scheduled_server.dart';
 import 'package:scheduled_test/descriptor.dart';
@@ skipping 119 matching lines @@
     if (versions is! List) versions = [versions];
     for (var version in versions) {
       var packageContents = [libDir(name, '$name $version')];
       if (includePubspecs) {
         packageContents.add(libPubspec(name, version));
       }
       contents.add(dir("$name-$version", packageContents));
     }
   });
 
+  return hostedCache(contents);
+}
+
+/// Describes the main cache directory containing cached hosted packages
+/// downloaded from the mock package server.
+Descriptor hostedCache(Iterable<Descriptor> contents) {
   return dir(cachePath, [
     dir('hosted', [
       async(port.then((p) => dir('127.0.0.1%58$p', contents)))
     ])
   ]);
 }
 
 /// Describes the file in the system cache that contains the client's OAuth2
 /// credentials. The URL "/token" on [server] will be used as the token
 /// endpoint for refreshing the access token.
@@ skipping 11 matching lines @@
           ['https://www.googleapis.com/auth/userinfo.email'],
           expiration).toJson())
     ]);
   }));
 }
 
 /// Describes the application directory, containing only a pubspec specifying
 /// the given [dependencies].
 DirectoryDescriptor appDir([Map dependencies]) =>
   dir(appPath, [appPubspec(dependencies)]);