Don't expose mangling/minification scheme in Symbol.toString.

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 be09eaef8c985eac7d3c929c2d0665a53378a4a3..35a0d88e95bb2b470eb52e111b4ca80643ff281e 100644
--- a/runtime/lib/symbol_patch.dart
+++ b/runtime/lib/symbol_patch.dart
@@ -2,7 +2,11 @@
 // 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:mirrors';
+
 patch class Symbol {
   /* patch */ const Symbol(String name)
       : this._name = name;
+
+  /* patch */ toString() => 'Symbol("${MirrorSystem.getName(this)}")';
 }