Support running tests without a packages/ directory.

test/test_pub.dart

Index: test/test_pub.dart
diff --git a/test/test_pub.dart b/test/test_pub.dart
index 24cd4407740fa35aa0bddd9f50018229d3b597af..835ef3e39616a0c223eecd7bf451a810d0ac5b4c 100644
--- a/test/test_pub.dart
+++ b/test/test_pub.dart
@@ -14,6 +14,7 @@ import 'dart:math';
 
 import 'package:async/async.dart';
 import 'package:http/testing.dart';
+import 'package:package_resolver/package_resolver.dart';
 import 'package:path/path.dart' as p;
 import 'package:pub/src/entrypoint.dart';
 import 'package:pub/src/exceptions.dart';
@@ -346,7 +347,7 @@ PubProcess startPub({List args, Future<String> tokenEndpoint,
   if (fileExists('$pubPath.snapshot')) pubPath += '.snapshot';
 
   var dartArgs = <dynamic>[
-    '--package-root=${p.toUri(p.absolute(p.fromUri(Platform.packageRoot)))}',
+    PackageResolver.current.processArgument,
     pubPath,
     '--verbose'
   ]..addAll(args);