title: Google Protobufs in Java: How do you get a Message.Builder from a FieldDescriptor?
body: <p>I have a FieldDescriptor for a message field defined in my protocol buffer.  I want to start constructing a value for that field, but I'm stuck trying to get a Message.Builder for that FieldDescriptor.  The code I'm writing is extremely generic - It's designed to serialize between MongoDB and Protocol Buffers - so I can't use any specialized logic for the Objects I happen to be using today.</p>
  
  <p>The FieldDescriptor's JavaType is MESSAGE.  It's MessageType is a bit better, as it contains the message's Type, but the Type is in protocol buffer namespace, so I still can't use reflection to get a Message for it (at least not cleanly).</p>
  
  <p>I'm not sure what else to do.  Anyone know how to construct a Message.Builder from a FieldDescriptor?</p>