Dropped bytes at serial inputs for up to 6.6ms after reading bytes via native USB

_From @EmbedHA on January 3, 2014 13:19_

On the Arduino DUE, using IDE 1.5.4, following issue was found.
For a time of up to ca. 6.6ms after a SerialUSB.readBytes statement, the reception of serial bytes is corrupted by randomly dropped bytes - mostly one byte, sometimes two. This was tested tested from 115200baud down to 28800baud for the serial input and for serial1 and serial2.

An extensive description of a setup to check the issue, sample code to demonstrate the problem, test logs etc. can be found in the Arduino forum: http://forum.arduino.cc/index.php?topic=207299.0

_Copied from original issue: arduino/Arduino#1784_