Make default argument to Iterable.join be "".

sdk/lib/io/http_impl.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 _HttpIncoming extends Stream<List<int>> {
   final int _transferLength;
   final Completer _dataCompleter = new Completer();
   Stream<List<int>> _stream;
@@ skipping 367 matching lines @@
     if (string.isEmpty) return;
     _ioSink.write(string);
   }
 
   void writeAll(Iterable objects, [String separator = ""]) {
     bool isFirst = true;
     for (Object obj in objects) {
       if (isFirst) {
         isFirst = false;
       } else {
-        if (separator != "") write(separator);
+        if (!separator.isEmpty) write(separator);
       }
       write(obj);
     }
   }
 
   void writeln([Object obj = ""]) {
     write(obj);
     write("\n");
   }
 
@@ skipping 1421 matching lines @@
 
 
 class _RedirectInfo implements RedirectInfo {
   const _RedirectInfo(int this.statusCode,
                       String this.method,
                       Uri this.location);
   final int statusCode;
   final String method;
   final Uri location;
 }