Add an http_throttle package.

pkg/http_throttle/lib/http_throttle.dart

Index: pkg/http_throttle/lib/http_throttle.dart
diff --git a/pkg/http_throttle/lib/http_throttle.dart b/pkg/http_throttle/lib/http_throttle.dart
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+++ b/pkg/http_throttle/lib/http_throttle.dart
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+// 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 http_throttle;
+
+import 'dart:async';
+
+import 'package:http/http.dart';
+import 'package:pool/pool.dart';
+
+/// A middleware client that throttles the number of concurrent requests.
+///
+/// As long as the number of requests is within the limit, this works just like
+/// a normal client. If a request is made beyond the limit, the underlying HTTP
+/// request won't be sent until other requests have completed.
+class ThrottleClient extends BaseClient {
+  final Pool _pool;
+  final Client _inner;
+
+  /// Creates a new client that allows no more than [maxActiveRequests]
+  /// concurrent requests.
+  ///
+  /// If [inner] is passed, it's used as the inner client for sending HTTP
+  /// requests. It defaults to `new http.Client()`.
+  ThrottleClient(int maxActiveRequests, [Client inner])
+      : _pool = new Pool(maxActiveRequests),
+        _inner = inner == null ? new Client() : inner;
+
+  Future<StreamedResponse> send(BaseRequest request) {
+    return _pool.request().then((resource) {
+      return _inner.send(request).then((response) {
+        var stream = response.stream.transform(
+            new StreamTransformer.fromHandlers(handleDone: (sink) {
+          resource.release();
+          sink.close();
+        }));
+        return new StreamedResponse(stream, response.statusCode,
+            contentLength: response.contentLength,
+            request: response.request,
+            headers: response.headers,
+            isRedirect: response.isRedirect,
+            persistentConnection: response.persistentConnection,
+            reasonPhrase: response.reasonPhrase);
+      });
+    });
+  }
+
+  void close() => _inner.close();
+}