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

sdk/lib/_internal/pub/test/cache/list_test.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.
 
 library pub_cache_test;
 
 import 'package:path/path.dart' as path;
 
-import 'descriptor.dart' as d;
-import 'test_pub.dart';
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
 
 main() {
   initConfig();
 
   hostedDir(package) {
     return path.join(sandboxDir, cachePath, "hosted",
         "pub.dartlang.org", package);
   }
 
   integration('running pub cache list when there is no cache', () {
@@ skipping 49 matching lines @@
       ])
     ]).create();
 
     schedulePub(args: ['cache', 'list'], outputJson: {
       "packages": {
         "foo": {"1.2.3": {"location": hostedDir('foo-1.2.3')}}
       }
     });
   });
 }