Support directories other than "web" in pub build.

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

+// Copyright (c) 2014, 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 '../../lib/src/exit_codes.dart' as exit_codes;
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+
+main() {
+  initConfig();
+
+  integration("fails if given directories are not buildable", () {
+    d.appDir().create();
+
+    schedulePub(args: ["build", "foo", "bar"],
+        error: 'Unsupported build directories "bar" and "foo".\n'
+               'The allowed directories are "benchmark", "bin", "example", '
+                   '"test" and "web".',
+        exitCode: exit_codes.USAGE);
+  });
+}