Add the package implementation.

lib/src/package_config_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.
+
+import 'package:package_resolver/package_resolver.dart';
+import 'package:path/path.dart' as p;
+import 'package:shelf/shelf.dart';
+import 'package:shelf_static/shelf_static.dart';
+
+import 'async_handler.dart';
+
+/// A shelf handler that serves a virtual packages directory based on a package
+/// config.
+class PackageConfigHandler {
+  /// The static handlers for serving entries in the package config, indexed by
+  /// name.
+  final _packageHandlers = new Map<String, Handler>();
+
+  /// The information specifying how to do package resolution.
+  PackageResolver _resolver;
+
+  PackageConfigHandler(this._resolver);
+
+  /// The callback for handling a single request.
+  call(Request request) {
+    var segments = request.url.pathSegments;
+    return _handlerFor(segments.first)(request.change(path: segments.first));
+  }
+
+  /// Creates a handler for [package] based on the package map in [resolver].
+  Handler _handlerFor(String package) {
+    return _packageHandlers.putIfAbsent(package, () {
+      return new AsyncHandler(_resolver.urlFor(package).then((url) {
+        var handler = url == null
+            ? (_) => new Response.notFound("Package $package not found.")
+            : createStaticHandler(p.fromUri(url), serveFilesOutsidePath: true);
+
+        return handler;
+      }));
+    });
+  }
+}