Make servePackages's contents argument saner.

sdk/lib/_internal/pub/test/global/activate/empty_constraint_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 '../../../lib/src/exit_codes.dart' as exit_codes;
 import '../../test_pub.dart';
 
 main() {
   initConfig();
   integration('errors if the constraint matches no versions', () {
-    servePackages([
-      packageMap("foo", "1.0.0"),
-      packageMap("foo", "1.0.1")
-    ]);
+    servePackages((builder) {
+      builder.serve("foo", "1.0.0");
+      builder.serve("foo", "1.0.1");
+    });
 
     schedulePub(args: ["global", "activate", "foo", ">1.1.0"],
         error: """
             Package foo has no versions that match >1.1.0 derived from:
             - pub global activate depends on version >1.1.0""",
         exitCode: exit_codes.DATA);
   });
 }