Reduce foot-print of Error.safeToString on strings.

runtime/lib/errors_patch.dart

Index: runtime/lib/errors_patch.dart
diff --git a/runtime/lib/errors_patch.dart b/runtime/lib/errors_patch.dart
index b65ae30fe9781545411ce8fae31d272e62f8a40d..5f9bd9f0cb58ab59d1a0683fbb4a0515a4b12216 100644
--- a/runtime/lib/errors_patch.dart
+++ b/runtime/lib/errors_patch.dart
@@ -3,12 +3,17 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:mirrors' show MirrorSystem;
+import 'dart:convert' show JSON;
 
 patch class Error {
   /* patch */ static String _objectToString(Object object) {
     return Object._toString(object);
   }
 
+  /* patch */ static String _stringToSafeString(String string) {
+    return JSON.encode(string);
+  }
+
   /* patch */ StackTrace get stackTrace => _stackTrace;
 
   StackTrace _stackTrace;