Refactor version_solver_test.

test/package_server.dart

Index: test/package_server.dart
diff --git a/test/package_server.dart b/test/package_server.dart
index bfe294058736322836a32f117648e39abd7c0c18..7561a5ae57192dad66d17186daa0bef985119d1a 100644
--- a/test/package_server.dart
+++ b/test/package_server.dart
@@ -74,6 +74,9 @@ class PackageServer {
   /// A future that will complete to the port used for the server.
   Future<int> get port => _inner.port;
 
+  /// A future that will complete to the URL for the server.
+  Future<String> get url async => 'http://localhost:${await port}';
+
   /// Creates an HTTP server that replicates the structure of pub.dartlang.org.
   ///
   /// Calls [callback] with a [PackageServerBuilder] that's used to specify