Added README, a bin/serve example server which can be used locally.

example/src/examples/http_proxy_repository.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.
+
+library appengine_pub.http_proxy_repository;
+
+import 'dart:async';
+import 'dart:convert';
+
+import 'package:http/http.dart' as http;
+import 'package:logging/logging.dart';
+import 'package:pub_server/repository.dart';
+
+final Logger _logger = new Logger('pub_server.http_proxy_repository');
+
+/// Implements the [PackageRepository] by talking to a remote HTTP server via
+/// the pub HTTP API.
+///
+/// This [PackageRepository] does not support uploading so far.
+class HttpProxyRepository extends PackageRepository {
+  final http.Client client;
+  final Uri baseUrl;
+
+  HttpProxyRepository(this.client, this.baseUrl);
+
+  Stream<PackageVersion> versions(String package) {
+    Future<List<PackageVersion>> fetch() async {
+      Uri versionUrl = baseUrl.resolve(
+          '/api/packages/${Uri.encodeComponent(package)}');
+
+      http.Response response = await client.get(versionUrl);
+      var json = JSON.decode(response.body);
+      var versions = json['versions'];
+      if (versions != null) {
+        return versions.map((Map item) {
+          var pubspec = item['pubspec'];
+          var pubspecString = JSON.encode(pubspec);
+          return new PackageVersion(
+              pubspec['name'], pubspec['version'], pubspecString);
+        }).toList();
+      }
+      return const [];
+    }
+
+    var controller = new StreamController();
+
+    fetch().then((List<PackageVersion> packageVersions) {
+      for (var packageVersion in packageVersions) {
+        controller.add(packageVersion);
+      }
+    }).catchError(controller.addError).whenComplete(controller.close);
+
+    return controller.stream;
+  }
+
+  // TODO: Could be optimized, since we don't need to list all versions and can
+  // just talk to the HTTP endpoint which gives us a specific package/version
+  // combination.
+  Future<PackageVersion> lookupVersion(String package, String version) {
+    return versions(package)
+        .where((v) => v.packageName == package && v.versionString == version)
+        .toList().then((List<PackageVersion> versions) {
+      if (versions.length >= 1) return versions.first;
+      return null;
+    });
+  }
+
+  bool get supportsUpload => false;
+
+  bool get supportsAsyncUpload => false;
+
+  bool get supportsDownloadUrl => true;
+
+  Future<Uri> downloadUrl(String package, String version) async {
+    package = Uri.encodeComponent(package);
+    version = Uri.encodeComponent(version);
+    return baseUrl.resolve('/packages/$package/versions/$version.tar.gz');
+  }
+
+  Future<Stream> download(String package, String version) async {
+    _logger.info('Downloading package $package/$version.');
+
+    var url = await downloadUrl(package, version);
+    var response = await client.send(new http.Request('GET', url));
+    return response.stream;
+  }
+}