Revert to throwing strings in JSON parsing.

lib/json/json.dart

 // Copyright (c) 2011, 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("json");
 
 // Pure Dart implementation of JSON protocol.
 
 /**
  * Utility class to parse JSON and serialize objects to JSON.
  */
 class JSON {
   /**
    * Parses [:json:] and build the corresponding object.
    */
   static parse(String json) {
     return _JsonParser.parse(json);
   }
 
   /**
    * Serializes [:object:] into JSON string.
    */
   static String stringify(Object object) {
     return JsonStringifier.stringify(object);
   }
 }
 
 //// Implementation ///////////////////////////////////////////////////////////
 
+// TODO(ajohnsen): Introduce when we have a common exception interface for json.
 class JSONParseException {
   JSONParseException(int position, String message) :
       position = position,
       message = 'JSONParseException: $message, at offset $position';
 
   String toString() => message;
 
   final String message;
   final int position;
 }
@@ skipping 302 matching lines @@
       if (token === WHITESPACE) {
         position++;
         continue;
       }
       if (token === null) _error("Invalid JSON token");
       return token;
     }
   }
 
   void _error(String message) {
-    throw new JSONParseException(position, message);
+    throw message;
   }
 
   final String json;
   int position = 0;
   static List<int> tokens;
 }
 
 // TODO: proper base class.
 class JsonUnsupportedObjectType {
   const JsonUnsupportedObjectType();
@@ skipping 144 matching lines @@
         });
         _sb.add('}');
         _seen.removeLast();
         return;
 
       default:
         throw const JsonUnsupportedObjectType();
     }
   }
 }