Freeze DEFAULT_ATTR_DATA to hard fail when not cloned
Freeze DEFAULT_ATTR_DATA to hard fail when not cloned

Tyriar/xterm.js,sourcelair/xterm.js,xtermjs/xterm.js,xtermjs/xterm.js,sourcelair/xterm.js,sourcelair/xterm.js,sourcelair/xterm.js,Tyriar/xterm.js,Tyriar/xterm.js,xtermjs/xterm.js,Tyriar/xterm.js,Tyriar/xterm.js,xtermjs/xterm.js,xtermjs/xterm.js