Support directories other than "web" in pub build.

sdk/lib/_internal/pub/test/build/name_arg_builds_only_that_directory_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 '../descriptor.dart' as d;
+import '../test_pub.dart';
+
+main() {
+  initConfig();
+
+  integration("if a dir name is given, only includes that dir", () {
+    d.dir(appPath, [
+      d.appPubspec(),
+      d.dir('asset', [
+        d.file('file.txt', 'asset')
+      ]),
+      d.dir('example', [
+        d.file('file.txt', 'example')
+      ]),
+      d.dir('web', [
+        d.file('file.txt', 'test')
+      ])
+    ]).create();
+
+    schedulePub(args: ["build", "example"],
+        output: new RegExp(r"Built 2 files!"));
+
+    d.dir(appPath, [
+      d.dir('build', [
+        d.dir('example', [
+          d.file('file.txt', 'example'),
+          d.dir('assets', [
+            d.dir('myapp', [
+              d.file('file.txt', 'asset')
+            ])
+          ])
+        ]),
+        // Only example should be built.
+        d.nothing('web')
+      ])
+    ]).validate();
+  });
+}