Report correct socket error and use HttpServer:close future in pub.

sdk/lib/_internal/pub/test/test_pub.dart

Index: sdk/lib/_internal/pub/test/test_pub.dart
diff --git a/sdk/lib/_internal/pub/test/test_pub.dart b/sdk/lib/_internal/pub/test/test_pub.dart
index 5d1afcfd17fb1b7ca35ac96e14068d3dde17a381..c54a3bdf9a86e0d469c1d3c9ec3927ddc3087583 100644
--- a/sdk/lib/_internal/pub/test/test_pub.dart
+++ b/sdk/lib/_internal/pub/test/test_pub.dart
@@ -10,7 +10,7 @@ library test_pub;
 
 import 'dart:async';
 import 'dart:collection' show Queue;
-import 'dart:io' hide sleep;
+import 'dart:io';
 import 'dart:json' as json;
 import 'dart:math';
 import 'dart:utf';
@@ -137,13 +137,10 @@ void serve([List<d.Descriptor> contents]) {
 /// is closed.
 Future _closeServer() {
   if (_server == null) return new Future.value();
-  _server.close();
+  var future = _server.close();
   _server = null;
   _portCompleterCache = null;
-  // TODO(nweiz): Remove this once issue 4155 is fixed. Pumping the event loop
-  // *seems* to be enough to ensure that the server is actually closed, but I'm
-  // putting this at 10ms to be safe.
-  return sleep(10);
+  return future;
 }
 
 /// `true` if the current test spins up an HTTP server.