Support directories other than "web" in pub build.

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

Index: sdk/lib/_internal/pub/test/build/missing_build_directories_test.dart
diff --git a/sdk/lib/_internal/pub/test/build/missing_build_directories_test.dart b/sdk/lib/_internal/pub/test/build/missing_build_directories_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..86e20570c88e125dbe5c51d8ff28193c5a282c0d
--- /dev/null
+++ b/sdk/lib/_internal/pub/test/build/missing_build_directories_test.dart
@@ -0,0 +1,26 @@
+// 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 '../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: 'Could not find directories "benchmark" and "test".',
+        exitCode: 1);
+  });
+}