I checked the syntax you posted for embedding json file is incorrect. Please make it correct, it should be like
[Embed(source="a.json", mimeType="application/octet-stream")] public var jsonData:Class;
Then you can access json data like this:-
var str:String = new String(new jsonData()); var obj:Object = JSON.parse(str);
Please make sure your a.json file must have a valid json data.