Make servePackages's contents argument saner.

sdk/lib/_internal/pub/test/downgrade/unlock_dependers_test.dart

 // Copyright (c) 2012, 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("downgrades a locked package's dependers in order to get it to "
       "min version", () {
-    servePackages([
-      packageMap("foo", "2.0.0", {"bar": ">1.0.0"}),
-      packageMap("bar", "2.0.0")
-    ]);
+    servePackages((builder) {
+      builder.serve("foo", "2.0.0", pubspec: {
+        "dependencies": {"bar": ">1.0.0"}
+      });
+      builder.serve("bar", "2.0.0");
+    });
 
     d.appDir({"foo": "any", "bar": "any"}).create();
 
     pubGet();
 
     d.packagesDir({
       "foo": "2.0.0",
       "bar": "2.0.0"
     }).validate();
 
-    servePackages([
-      packageMap("foo", "1.0.0", {"bar": "any"}),
-      packageMap("bar", "1.0.0")
-    ]);
+    servePackages((builder) {
+      builder.serve("foo", "1.0.0", pubspec: {
+        "dependencies": {"bar": "any"}
+      });
+      builder.serve("bar", "1.0.0");
+    });
 
     pubDowngrade(args: ['bar']);
 
     d.packagesDir({
       "foo": "1.0.0",
       "bar": "1.0.0"
     }).validate();
   });
 }