Support serving from multiple directories using "pub serve".

sdk/lib/_internal/pub/test/serve/roots/throws_an_error_if_custom_roots_dont_exist_test.dart

Index: sdk/lib/_internal/pub/test/serve/roots/throws_an_error_if_custom_roots_dont_exist_test.dart
diff --git a/sdk/lib/_internal/pub/test/serve/roots/throws_an_error_if_custom_roots_dont_exist_test.dart b/sdk/lib/_internal/pub/test/serve/roots/throws_an_error_if_custom_roots_dont_exist_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..c2275bbc16d16fe6c8788301726fb701447806e9
--- /dev/null
+++ b/sdk/lib/_internal/pub/test/serve/roots/throws_an_error_if_custom_roots_dont_exist_test.dart
@@ -0,0 +1,27 @@
+// Copyright (c) 2014, 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 '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();
+  integration("throws an error if custom roots don't exist", () {
+    d.dir(appPath, [
+      d.appPubspec(),
+      d.dir("baz")
+    ]).create();
+
+    var server = startPubServe(args: ["foo", "bar", "baz"]);
+    server.stderr.expect('Directories "bar" and "foo" don\'t exist.');
+    server.shouldExit(exit_codes.USAGE);
+  });
+}