Reduce foot-print of Error.safeToString on strings.

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

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // 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.
 
 library _js_helper;
 
 import 'shared/embedded_names.dart' show
     ALL_CLASSES,
     GET_ISOLATE_TAG,
     INTERCEPTED_NAMES,
@@ skipping 3220 matching lines @@
  *
  * This will be truncated to the 53 bits available in a double.
  */
 int random64() {
   // TODO(lrn): Use a secure random source.
   int int32a = JS("int", "(Math.random() * 0x100000000) >>> 0");
   int int32b = JS("int", "(Math.random() * 0x100000000) >>> 0");
   return int32a + int32b * 0x100000000;
 }
 
+String jsonEncodeNative(String string) {
+  return JS("String", "JSON.stringify(#)", string);
+}
+
 /**
  * Returns a property name for placing data on JavaScript objects shared between
  * DOM isolates.  This happens when multiple programs are loaded in the same
  * JavaScript context (i.e. page).  The name is based on [name] but with an
  * additional part that is unique for each isolate.
  *
  * The form of the name is '___dart_$name_$id'.
  */
 String getIsolateAffinityTag(String name) {
   var isolateTagGetter =
@@ skipping 147 matching lines @@
   throw new MainError("No top-level function named 'main'.");
 }
 
 void badMain() {
   throw new MainError("'main' is not a function.");
 }
 
 void mainHasTooManyParameters() {
   throw new MainError("'main' expects too many parameters.");
 }