Use browsers JSON.parse for parsing JSON (#3)

sdk/lib/json/json_base.dart

Index: sdk/lib/json/json_base.dart
diff --git a/sdk/lib/json/json_base.dart b/sdk/lib/json/json_base.dart
index 9f74d6585b22a5959f4a3b5348777a679b85addc..bf1b01e6fd6b8667e0e3c57395a50b3ff68d81be 100644
--- a/sdk/lib/json/json_base.dart
+++ b/sdk/lib/json/json_base.dart
@@ -47,7 +47,9 @@ class JsonUnsupportedObjectError implements Error {
  *
  * Throws [FormatException] if the input is not valid JSON text.
  */
-parse(String json, [reviver(var key, var value)]) {
+external parse(String json, [reviver(var key, var value)]);
+
+_parse(String json, reviver(var key, var value)) {
   BuildJsonListener listener;
   if (reviver == null) {
     listener = new BuildJsonListener();