MXMLC throws error for JSON object, Flash Builder OK

I have some code in a project that always compiled fine in Flash Builder 4.5 (using the 4.1 SDK).  I am trying to build it with the command-line build using Sublime Text 2, and it fails on this code:

                                        var objSend:Object = new Object();                                        var dataString:String = JSON.encode(packagedData);                                        dataString = escape(dataString);                                        objSend.jsonSendData = dataString;

It gives an error on that second line:

Error: Access of undefined property JSON.

Why?  Is there anything I need to include?