Don't load assets unnecessarily when precompiling executables.

sdk/lib/_internal/pub/test/no_package_symlinks_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:scheduled_test/scheduled_test.dart';
 
 import 'descriptor.dart' as d;
 import 'test_pub.dart';
 
 main() {
   initConfig();
 
   forBothPubGetAndUpgrade((command) {
     group("with --no-package-symlinks", () {
       integration("installs hosted dependencies to the cache", () {
-        servePackages([
-          packageMap("foo", "1.0.0"),
-          packageMap("bar", "1.0.0")
-        ]);
+        servePackages((builder) {
+          builder.serve("foo", "1.0.0");
+          builder.serve("bar", "1.0.0");
+        });
 
         d.appDir({"foo": "any", "bar": "any"}).create();
 
         pubCommand(command, args: ["--no-package-symlinks"]);
 
         d.nothing("$appPath/packages").validate();
 
         d.hostedCache([
           d.dir("foo-1.0.0", [
             d.dir("lib", [d.file("foo.dart", 'main() => "foo 1.0.0";')])
@@ skipping 76 matching lines @@
 
         d.dir(appPath, [
           d.nothing("packages"),
           d.dir("bin/subdir/packages"),
           d.dir("lib/packages")
         ]).validate();
       });
     });
   });
 }