Add a check to JSON code

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.
@@ skipping 89 matching lines @@
   static final String TRUE_STRING = "true";
   static final String FALSE_STRING = "false";
 
 
   static parse(String json) {
     return new _JsonParser._internal(json)._parseToplevel();
   }
 
   static objectLength(String str) {
     var p = new _JsonParser._internal(str);
+    var firstToken = p._token();
+    if (firstToken != LBRACE) {
+      return 0;
+    }
     try {
       p._parseObject();
       assert(p.position <= p.length);
       return p.position;
     } catch (var e) {
       return 0;
     }
   }
 
   _JsonParser._internal(String json)
@@ skipping 420 matching lines @@
         });
         _sb.add('}');
         _seen.removeLast();
         return;
 
       default:
         throw const JsonUnsupportedObjectType();
     }
   }
 }