Handle assignability for Send and SendSet

sdk/lib/io/http_headers.dart

 // Copyright (c) 2013, 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.
 
 part of dart.io;
 
 class _HttpHeaders implements HttpHeaders {
   _HttpHeaders(String this.protocolVersion)
       : _headers = new Map<String, List<String>>();
 
   List<String> operator[](String name) {
     name = name.toLowerCase();
     return _headers[name];
   }
 
   String value(String name) {
     name = name.toLowerCase();
     List<String> values = _headers[name];
     if (values == null) return null;
     if (values.length > 1) {
       throw new HttpException("More than one value for header $name");
     }
     return values[0];
   }
 
-  void add(String name, Object value) {
+  void add(String name, value) {
     _checkMutable();
     if (value is List) {
       for (int i = 0; i < value.length; i++) {
         _add(name, value[i]);
       }
     } else {
       _add(name, value);
     }
   }
 
@@ skipping 734 matching lines @@
 
   String name;
   String value;
   DateTime expires;
   int maxAge;
   String domain;
   String path;
   bool httpOnly = false;
   bool secure = false;
 }