Add the package implementation.

lib/shelf_packages_handler.dart

Index: lib/shelf_packages_handler.dart
diff --git a/lib/shelf_packages_handler.dart b/lib/shelf_packages_handler.dart
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index 0000000000000000000000000000000000000000..eda62032d7831f62bd9294c9aaefeade91e48605
--- /dev/null
+++ b/lib/shelf_packages_handler.dart
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+// 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;
+
+import 'package:shelf/shelf.dart';
+import 'package:shelf_static/shelf_static.dart';
+import 'package:package_resolver/package_resolver.dart';
+import 'package:path/path.dart' as p;
+
+import 'src/async_handler.dart';
+import 'src/dir_handler.dart';
+import 'src/package_config_handler.dart';
+
+/// A handler that serves the contents of a virtual packages directory.
+///
+/// This effectively serves `package:${request.url}`. It locates packages using
+/// the package resolution logic defined by [resolver]. If [resolver] isn't
+/// passed, it defaults to the current isolate's package resolution logic.
+///
+/// This can only serve assets from `file:` URIs.
+Handler packagesHandler({PackageResolver resolver}) {
+  resolver ??= PackageResolver.current;
+  return new AsyncHandler(resolver.packageRoot.then((packageRoot) {
+    if (packageRoot != null) {
+      return createStaticHandler(p.fromUri(packageRoot),
+          serveFilesOutsidePath: true);
+    } else {
+      return new PackageConfigHandler(resolver);
+    }
+  }));
+}
+
+/// A handler that serves virtual `packages/` directories wherever they're
+/// requested.
+///
+/// This serves the same assets as [packagesHandler] for every URL that contains
+/// `/packages/`. Otherwise, it returns 404s for all requests.
+///
+/// This is useful for ensuring that `package:` imports work for all entrypoints
+/// in Dartium.
+Handler packagesDirHandler({PackageResolver resolver}) =>
+    new DirHandler("packages", packagesHandler(resolver: resolver));