Support directories other than "web" in pub build.

sdk/lib/_internal/pub/test/build/dart2js_finds_imports_across_packages_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 '../descriptor.dart' as d;
 import '../test_pub.dart';
 
 main() {
@@ skipping 29 matching lines @@
       d.dir("web", [
         d.file("main.dart",
             """
             import 'package:foo/foo.dart';
             main() => print(foo());
             """)
       ])
     ]).create();
 
     schedulePub(args: ["build"],
-        output: new RegExp(r"Built 3 files!"),
-        exitCode: 0);
+        output: new RegExp(r"Built 3 files!"));
 
     d.dir(appPath, [
       d.dir('build', [
-        d.matcherFile('main.dart.js', isNot(isEmpty)),
-        d.matcherFile('main.dart.precompiled.js', isNot(isEmpty)),
-        d.matcherFile('main.dart.js.map', isNot(isEmpty))
+        d.dir('web', [
+          d.matcherFile('main.dart.js', isNot(isEmpty)),
+          d.matcherFile('main.dart.precompiled.js', isNot(isEmpty)),
+          d.matcherFile('main.dart.js.map', isNot(isEmpty))
+        ])
       ])
     ]).validate();
   });
 }