Add optional writeAll separator argument.

pkg/http/test/safe_http_server.dart

Index: pkg/http/test/safe_http_server.dart
diff --git a/pkg/http/test/safe_http_server.dart b/pkg/http/test/safe_http_server.dart
index 64adf5b8ac691f4daee8c7c56489300a8478db99..01df2d93ed621654bea91e47f85d1fef59629a17 100644
--- a/pkg/http/test/safe_http_server.dart
+++ b/pkg/http/test/safe_http_server.dart
@@ -138,7 +138,8 @@ class _HttpResponseWrapper implements HttpResponse {
     _inner.writeStream(stream);
   void close() => _inner.close();
   void write(Object obj) => _inner.write(obj);
-  void writeAll(Iterable objects) => _inner.writeAll(objects);
+  void writeAll(Iterable objects, [String separator = ""]) =>
+      _inner.writeAll(objects, separator);
   void writeCharCode(int charCode) => _inner.writeCharCode(charCode);
   void writeln([Object obj = ""]) => _inner.writeln(obj);
 }