Don't load assets unnecessarily when precompiling executables.

sdk/lib/_internal/pub/test/cache/add/already_cached_test.dart

 // Copyright (c) 2014, 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_tests;
 
 import '../../descriptor.dart' as d;
 import '../../test_pub.dart';
 
 main() {
   initConfig();
   integration('does nothing if the package is already cached', () {
-    servePackages([
-      packageMap("foo", "1.2.3")
-    ]);
+    servePackages((builder) {
+      builder.serve("foo", "1.2.3");
+    });
 
     // Run once to put it in the cache.
     schedulePub(args: ["cache", "add", "foo"],
         output: 'Downloading foo 1.2.3...');
 
     // Should be in the cache now.
     schedulePub(args: ["cache", "add", "foo"],
         output: 'Already cached foo 1.2.3.');
 
     d.cacheDir({"foo": "1.2.3"}).validate();
   });
 }