Revert to throwing strings in JSON parsing.

lib/json/json.dart

Index: lib/json/json.dart
diff --git a/lib/json/json.dart b/lib/json/json.dart
index 523d3cdc2d572cb00fbae5b86b5b06f777a6d02e..9448b58707981651b957ed31af89abb38065ea02 100644
--- a/lib/json/json.dart
+++ b/lib/json/json.dart
@@ -27,6 +27,7 @@ class JSON {
 
 //// Implementation ///////////////////////////////////////////////////////////
 
+// TODO(ajohnsen): Introduce when we have a common exception interface for json.
 class JSONParseException {
   JSONParseException(int position, String message) :
       position = position,
@@ -349,7 +350,7 @@ class _JsonParser {
   }
 
   void _error(String message) {
-    throw new JSONParseException(position, message);
+    throw message;
   }
 
   final String json;