Revert "Remove support for "packages" URL from testing server."

tools/testing/dart/configuration.dart

 // Copyright (c) 2017, 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 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
 import 'compiler_configuration.dart';
 import 'http_server.dart';
@@ skipping 352 matching lines @@
 
     return isValid;
   }
 
   /// Starts global HTTP servers that serve the entire dart repo.
   ///
   /// The HTTP server is available on `window.location.port`, and a second
   /// server for cross-domain tests can be found by calling
   /// `getCrossOriginPortNumber()`.
   Future startServers() {
-    _servers = new TestingServers(buildDirectory, isCsp, runtime, null);
+    _servers = new TestingServers(
+        buildDirectory, isCsp, runtime, null, packageRoot, packages);
     var future = servers.startServers(localIP,
         port: testServerPort, crossOriginPort: testServerCrossOriginPort);
 
     if (isVerbose) {
       future = future.then((_) {
         print('Started HttpServers: ${servers.httpServerCommandLine()}');
       });
     }
 
     return future;
@@ skipping 347 matching lines @@
 
       case macos:
         return 'xcodebuild/';
     }
 
     throw "unreachable";
   }
 
   String toString() => "System($name)";
 }