Don't load assets unnecessarily when precompiling executables.

sdk/lib/_internal/pub/test/global/run/nonexistent_script_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.
 
 import 'package:path/path.dart' as p;
 
 import '../../../lib/src/exit_codes.dart' as exit_codes;
 import '../../test_pub.dart';
 
 main() {
   initConfig();
   integration('errors if the script does not exist.', () {
-    servePackages([packageMap("foo", "1.0.0")]);
+    servePackages((builder) => builder.serve("foo", "1.0.0"));
 
     schedulePub(args: ["global", "activate", "foo"]);
 
     var pub = pubRun(global: true, args: ["foo:script"]);
     pub.stderr.expect("Could not find ${p.join("bin", "script.dart")}.");
     pub.shouldExit(exit_codes.NO_INPUT);
   });
 }