Add the package implementation.

lib/src/dir_handler.dart

+// Copyright (c) 2016, 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.
+
+library shelf_packages_handler.dir_handler;
+
+import 'package:path/path.dart' as p;
+import 'package:shelf/shelf.dart';
+
+/// A utility handler that mounts a sub-handler beneath a directory name,
+/// wherever that directory name appears in a URL.
+///
+/// In practice, this is used to mount a [PackagesHandler] underneath
+/// `packages/` directories.
+class DirHandler {
+  /// The directory name to look for.
+  final String _name;
+
+  /// The inner handler to mount.
+  final Handler _inner;
+
+  DirHandler(this._name, this._inner);
+
+  /// The callback for handling a single request.
+  call(Request request) {
+    var segments = request.url.pathSegments;
+    for (var i = 0; i < segments.length; i++) {
+      if (segments[i] != _name) continue;
+      return _inner(request.change(path: p.url.joinAll(segments.take(i + 1))));
+    }
+
+    return new Response.notFound("Not found.");
+  }
+}