feature: tsconfig: es2015 -> es2017
feature: tsconfig: es2015 -> es2017

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