SAM HardwareSerial.h missing availableForWrite

I noticed that SAM UARTClass.h contains availableForWrite, but HardwareSerial.h does not. So could you add
virtual int availableForWrite(void) = 0; to SAM HardwareSerial.h
so I can use it as
HardwareSerial *port;
...
port=&Serial3
...
if ( port->availableForWrite()>0 ) ...

Start in the SAM HardwareSerial.h interface and compare it with the availableForWrite implementation already present in SAM UARTClass.h. Confirm that the HardwareSerial interface exposes the capability needed by HardwareSerial pointers, then verify the SAM core still builds and the example call through Serial3 is supported.

Written by the indexing model from the issue text.