Add an http_throttle package.

pkg/http_throttle/README.md

+`http_throttle` is middleware for the [http package][] that throttles the number
+of concurrent requests that an HTTP client can make.
+
+```dart
+// This client allows 32 concurrent requests.
+final client = new ThrottleClient(32);
+
+Future<List<String>> readAllUrls(Iterable<Uri> urls) {
+  return Future.wait(urls.map((url) {
+    // You can safely call as many client methods as you want concurrently, and
+    // ThrottleClient will ensure that only 32 underlying HTTP requests will be
+    // open at once.
+    return client.read(url);
+  }));
+}
+```
+
+[http package]: pub.dartlang.org/packages/http