Add implementations for Headers, Request, Response and dart:io/dart:html clients to http_base

dart/pkg/http_base/lib/http_base_io.dart

+library http_base.http_base_io;
+
+import 'dart:io' as io;
+import 'dart:async';
+
+import 'http_base.dart';
+export 'http_base.dart';
+
+/// An implementation for [RequestHandler]. It uses dart:io to make http
+/// requests.
+class Client {
+  // TODO: Should we provide a mechanism to close (forcefully or not) [_client]?
+  final io.HttpClient _client = new io.HttpClient();
+
+  Future<Response> call(Request request) {
+   return _client.openUrl(request.method, request.url).then((ioRequest) {
+     // TODO: Special case Cookie/Set-Cookie here!
+
+     for (var name in request.headers.names) {
+       ioRequest.headers.set(name, request.headers[name]);
+     }
+
+     var stream = request.read();
+     return ioRequest.addStream(stream).then((_) {
+       return ioRequest.close();
+     });
+   }).then((io.HttpClientResponse ioResponse) {
+     var headerMap = {};
+     ioResponse.headers.forEach((name, values) {
+       headerMap[name] = values;
+     });
+     var headers = new HeadersImpl(headerMap);
+
+     return new ResponseImpl(
+         ioResponse.statusCode, headers: headers, body: ioResponse);
+   });
+  }
+}