core/src/main/java/com/twitter/elephantbird/util/ThriftToDynamicProto.java
private Message.Builder mapEntryProtoBuilder(TStructDescriptor descriptor, Field field) {
    return messageBuilderMap.get(mapProtoMessageType(descriptor, field)).clone();
  }
private Message.Builder mapEntryProtoBuilder(TStructDescriptor descriptor, Field field) {
    return messageBuilderMap.get(mapProtoMessageType(descriptor, field)).clone();
  }
Return Protobufs builder for a Map field