Clear http session object on destroy.

sdk/lib/io/http_impl.dart

Index: sdk/lib/io/http_impl.dart
diff --git a/sdk/lib/io/http_impl.dart b/sdk/lib/io/http_impl.dart
index fcb55bca2bb4e082dd0b4a28ad50f6b6c6cc03b8..ee855ec8661e76acc5bd920272ab9938bce258f8 100644
--- a/sdk/lib/io/http_impl.dart
+++ b/sdk/lib/io/http_impl.dart
@@ -128,6 +128,12 @@ class _HttpRequest extends _HttpInboundMessage implements HttpRequest {
 
   HttpSession get session {
     if (_session != null) {
+      if (_session._destroyed) {
+        // It's destroyed, clear it.
+        _session = null;
+        // Create new session object by calling recursive.
+        return session;
+      }
       // It's already mapped, use it.
       return _session;
     }