Allow analysis server integration tests to run without package root.

pkg/analysis_server/test/integration/integration_tests.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.
 
 library test.integration.analysis;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:convert';
 import 'dart:io';
@@ skipping 498 matching lines @@
     // executable, and the package root into a shell script.
     String dartBinary = Platform.executable;
     String scriptDir = dirname(Platform.script.toFilePath(windows:
         Platform.isWindows));
     String serverPath = normalize(join(scriptDir, '..', '..', 'bin',
         'server.dart'));
     List<String> arguments = [];
     if (debugServer) {
       arguments.add('--debug');
     }
-    arguments.add('--package-root=${Platform.packageRoot}');
+    if (Platform.packageRoot.isNotEmpty) {
+      arguments.add('--package-root=${Platform.packageRoot}');
+    }
     arguments.add(serverPath);
     return Process.start(dartBinary, arguments).then((Process process) {
       Server server = new Server._(process);
       process.stdout.transform((new Utf8Codec()).decoder).transform(
           new LineSplitter()).listen((String line) {
         String trimmedLine = line.trim();
         server._recordStdio('RECV: $trimmedLine');
         var message;
         try {
           message = JSON.decoder.convert(trimmedLine);
@@ skipping 121 matching lines @@
    * Record a message that was exchanged with the server, and print it out if
    * [debugStdio] has been called.
    */
   void _recordStdio(String line) {
     if (_debuggingStdio) {
       print(line);
     }
     _recordedStdio.add(line);
   }
 }