Keep track of when a socket has been destroyed

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 32e03736b08da69347c3be8699814a5dd4b7e9bd..583380707bac10730eb967e4e4e404d442873fe2 100644
--- a/sdk/lib/io/http_impl.dart
+++ b/sdk/lib/io/http_impl.dart
@@ -1759,10 +1759,13 @@ class _HttpServer extends Stream<HttpRequest> implements HttpServer {
                                      cancelOnError: cancelOnError);
   }
 
-  void close() {
+  Future close() {
     closed = true;
+    Future result;
     if (_serverSocket != null && _closeServer) {
-      _serverSocket.close();
+      result = _serverSocket.close();
+    } else {
+      result = new Future.value();
     }
     if (_sessionManagerInstance != null) {
       _sessionManagerInstance.close();
@@ -1772,6 +1775,7 @@ class _HttpServer extends Stream<HttpRequest> implements HttpServer {
       connection.destroy();
     }
     _connections.clear();
+    return result;
   }
 
   int get port {