Add 'Server' and 'User-Agent' default header fields.

sdk/lib/io/http.dart

Index: sdk/lib/io/http.dart
diff --git a/sdk/lib/io/http.dart b/sdk/lib/io/http.dart
index 4ea77f72d75e5591bcb105d792c70036babb3d83..63dc3ac998981865d2ffa6667f30b92a5a47fa34 100644
--- a/sdk/lib/io/http.dart
+++ b/sdk/lib/io/http.dart
@@ -171,6 +171,16 @@ abstract class HttpServer implements Stream<HttpRequest> {
    * current connections handled by the server.
    */
   HttpConnectionsInfo connectionsInfo();
+
+  /**
+   * Set and get the default value of the `Server` header for all responses
+   * generated by this [HttpServer]. The default value is
+   * `Dart/<version> (dart:io)`.
+   *
+   * If the serverHeader is set to `null`, no default `Server` header will be
+   * added to each response.
+   */
+  String serverHeader;
 }
 
 
@@ -879,6 +889,16 @@ abstract class HttpClient {
    */
   Duration idleTimeout;
 
+  /**
+   * Set and get the default value of the `User-Agent` header for all requests
+   * generated by this [HttpClient]. The default value is
+   * `Dart/<version> (dart:io)`.
+   *
+   * If the userAgent is set to `null`, no default `User-Agent` header will be
+   * added to each request.
+   */
+  String userAgent;
+
   factory HttpClient() => new _HttpClient();
 
   /**