Fix pub path on Windows

Author: kevmoo

webdev/test/integration_test.dart

@@ -10,7 +10,7 @@ import 'package:test/test.dart';
 import 'package:test_descriptor/test_descriptor.dart' as d;
 import 'package:test_process/test_process.dart';
 
-final _webdevBin = p.absolute('bin/webdev.dart');
+final _webdevBin = p.absolute(p.join('bin', 'webdev.dart'));
 
 /// The path to the root directory of the SDK.
 final String _sdkDir = (() {