Support directories other than "web" in pub build.

sdk/lib/_internal/pub/test/build/reports_dart_parse_errors_test.dart

 // Copyright (c) 2013, 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 'package:scheduled_test/scheduled_test.dart';
 
 import '../../lib/src/exit_codes.dart' as exit_codes;
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 
@@ skipping 23 matching lines @@
         error: new RegExp(
             r"(?=(.|\n)*^Error on line 1 of .*[/\\]file\.dart:)"
             r"(.|\n)*^Error on line 1 of .*[/\\]subfile\.dart:",
             multiLine: true),
         output: new RegExp(r"Building myapp\.\.\.*"),
         exitCode: exit_codes.DATA);
 
     // Doesn't output anything if an error occurred.
     d.dir(appPath, [
       d.dir('build', [
-        d.nothing('file.dart.js'),
-        d.nothing('file.dart'),
-        d.nothing('subdir')
+        d.nothing('web')
       ])
     ]).validate();
   });
 }