Fix more lint style issues in runtime patches.

runtime/lib/symbol_patch.dart

Index: runtime/lib/symbol_patch.dart
diff --git a/runtime/lib/symbol_patch.dart b/runtime/lib/symbol_patch.dart
index ee52cfca6641c4ad64bd2b8bf890b10890873901..aabf923d2f7f2e85ae7022bf3122113db95553fe 100644
--- a/runtime/lib/symbol_patch.dart
+++ b/runtime/lib/symbol_patch.dart
@@ -2,6 +2,8 @@
 // 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.
 
+import 'dart:core' hide Symbol;
+
 @patch class Symbol {
   @patch const Symbol(String name)
       : this._name = name;