Reduce foot-print of Error.safeToString on strings.

sdk/lib/_internal/compiler/js_lib/core_patch.dart

Index: sdk/lib/_internal/compiler/js_lib/core_patch.dart
diff --git a/sdk/lib/_internal/compiler/js_lib/core_patch.dart b/sdk/lib/_internal/compiler/js_lib/core_patch.dart
index 77098e1dadfa834f46d625c466c2d74e9dd9d711..2e2bab921b565e4bfb634f39b82ebd753b15016c 100644
--- a/sdk/lib/_internal/compiler/js_lib/core_patch.dart
+++ b/sdk/lib/_internal/compiler/js_lib/core_patch.dart
@@ -8,6 +8,7 @@ import 'dart:_interceptors';
 import 'dart:_js_helper' show patch,
                               checkInt,
                               getRuntimeType,
+                              jsonEncodeNative,
                               JSSyntaxRegExp,
                               Primitives,
                               stringJoinUnchecked,
@@ -141,6 +142,11 @@ class Error {
   }
 
   @patch
+  static String _stringToSafeString(String string) {
+    return jsonEncodeNative(string);
+  }
+
+  @patch
   StackTrace get stackTrace => Primitives.extractStackTrace(this);
 }