Support directories other than "web" in pub build.

sdk/lib/_internal/pub/test/build/missing_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 any specified build directories don't exist", () {
+    d.dir(appPath, [
+      d.appPubspec(),
+      d.dir('example', [
+        d.file('file.txt', 'example')
+      ]),
+      d.dir('web', [
+        d.file('file.txt', 'test')
+      ])
+    ]).create();
+
+    schedulePub(args: ["build", "benchmark", "example", "test", "web"],
+        error: 'Directories "benchmark" and "test" do not exist.',
+        exitCode: exit_codes.DATA);
+  });
+}