stable and es6 code version produce different output

Is there something missing in the es6 version to make it work?

board information:

>process.env
={
  VERSION: "2v08.117",
  GIT_COMMIT: "8af54959a",
  BOARD: "LINUX",
  FLASH: 262144, SPIFLASH: 262144, STORAGE: 262144, RAM: 0,
  SERIAL: "00000000-addeadde-addeadde",
  CONSOLE: "Telnet",
  MODULES: "Flash,Storage,hea" ... "crypto,tensorflow",
  EXPTR: 22978688 }

code:

// stable code version
Modules.addCached("a",( function(){
  var _a,
      a = function(){_a = this;},
      pa = a.prototype;
  pa.sqrt = (a) => {return _a._sqrt(a);};
  pa._sqrt = (a) => {return Math.sqrt(a);};
  exports.create = function(){ return new a();};
}));
a = require('a').create();
console.log(a.sqrt(3));

// es6 code version
Modules.addCached("b",(()=>{
  var _b,
      b = () => {_b = this;},
      pb = b.prototype;
  pb.sqrt = (a) => {return _b._sqrt(a);};
  pb._sqrt = (a) => {return Math.sqrt(a);};
  exports.create = () => { return new b();};
}));
b = require('b').create();
console.log(b.sqrt(3));

output:

>1.73205080756
Uncaught Error: Function "_sqrt" not found!
 at line 1 col 4
_b._sqrt(a);
   ^
in function "sqrt" called from line 26 col 21
console.log(b.sqrt(3));
                    ^