Rationalize arg handling for pub build and serve.

sdk/lib/_internal/pub/test/barback/fails_on_disallowed_directories_test.dart

Index: sdk/lib/_internal/pub/test/barback/fails_on_disallowed_directories_test.dart
diff --git a/sdk/lib/_internal/pub/test/barback/fails_on_disallowed_directories_test.dart b/sdk/lib/_internal/pub/test/barback/fails_on_disallowed_directories_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..11cf6571188fe62fe953ab57d3256b58853c46eb
--- /dev/null
+++ b/sdk/lib/_internal/pub/test/barback/fails_on_disallowed_directories_test.dart
@@ -0,0 +1,25 @@
+// 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 'package:path/path.dart' as path;
+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';
+import 'utils.dart';
+
+main() {
+  initConfig();
+
+  setUp(() {
+    d.appDir().create();
+  });
+
+  var libSub = path.join("lib", "sub");
+  pubBuildAndServeShouldFail("if given directories are not allowed",
+      args: [libSub, "asset"],
+      error: 'Directories "$libSub" and "asset" are not allowed.',
+      exitCode: exit_codes.USAGE);
+}