Support directories other than "web" in pub build.

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

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS d.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 pub_tests;
 
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 
 const TRANSFORMER = """
@@ skipping 33 matching lines @@
       ])
     ]).create();
 
     createLockFile('myapp', pkg: ['barback']);
 
     schedulePub(args: ["build", "--mode", "depeche"],
         exitCode: 0);
 
     d.dir(appPath, [
       d.dir('build', [
-        d.file('foo.out', 'depeche')
+        d.dir('web', [
+          d.file('foo.out', 'depeche')
+        ])
       ])
     ]).validate();
   });
 }