Example of rollup buble conversion.

GridHelper.bubled.js

/**
 * @author mrdoob / http://mrdoob.com/
 */

var GridHelper = /*@__PURE__*/(function (LineSegments) {
  function GridHelper ( size, divisions, color1, color2 ) {

    size = size || 10;
    divisions = divisions || 10;
    color1 = new Color( color1 !== undefined ? color1 : 0x444444 );
    color2 = new Color( color2 !== undefined ? color2 : 0x888888 );

    var center = divisions / 2;
    var step = size / divisions;
    var halfSize = size / 2;

    var vertices = [], colors = [];

    for ( var i = 0, j = 0, k = - halfSize; i <= divisions; i ++, k += step ) {

      vertices.push( - halfSize, 0, k, halfSize, 0, k );
      vertices.push( k, 0, - halfSize, k, 0, halfSize );

      var color = i === center ? color1 : color2;

      color.toArray( colors, j ); j += 3;
      color.toArray( colors, j ); j += 3;
      color.toArray( colors, j ); j += 3;
      color.toArray( colors, j ); j += 3;

    }

    var geometry = new BufferGeometry();
    geometry.setAttribute( 'position', new Float32BufferAttribute( vertices, 3 ) );
    geometry.setAttribute( 'color', new Float32BufferAttribute( colors, 3 ) );

    var material = new LineBasicMaterial( { vertexColors: true, toneMapped: false } );

    LineSegments.call( this, geometry, material );

    this.type = 'GridHelper';

  }

  if ( LineSegments ) GridHelper.__proto__ = LineSegments;
  GridHelper.prototype = Object.create( LineSegments && LineSegments.prototype );
  GridHelper.prototype.constructor = GridHelper;

  GridHelper.prototype.copy = function copy ( source ) {

    LineSegments.prototype.copy.call( this, source );

    this.geometry.copy( source.geometry );
    this.material.copy( source.material );

    return this;

  };

  GridHelper.prototype.clone = function clone () {

    return new GridHelper().copy( this );

  };

  return GridHelper;
}(LineSegments));

GridHelper.original.js

/**
 * @author mrdoob / http://mrdoob.com/
 */

import { LineSegments } from '../objects/LineSegments.js';
import { LineBasicMaterial } from '../materials/LineBasicMaterial.js';
import { Float32BufferAttribute } from '../core/BufferAttribute.js';
import { BufferGeometry } from '../core/BufferGeometry.js';
import { Color } from '../math/Color.js';

class GridHelper extends LineSegments {

	constructor ( size, divisions, color1, color2 ) {
	
		size = size || 10;
		divisions = divisions || 10;
		color1 = new Color( color1 !== undefined ? color1 : 0x444444 );
		color2 = new Color( color2 !== undefined ? color2 : 0x888888 );

		var center = divisions / 2;
		var step = size / divisions;
		var halfSize = size / 2;

		var vertices = [], colors = [];

		for ( var i = 0, j = 0, k = - halfSize; i <= divisions; i ++, k += step ) {

			vertices.push( - halfSize, 0, k, halfSize, 0, k );
			vertices.push( k, 0, - halfSize, k, 0, halfSize );

			var color = i === center ? color1 : color2;

			color.toArray( colors, j ); j += 3;
			color.toArray( colors, j ); j += 3;
			color.toArray( colors, j ); j += 3;
			color.toArray( colors, j ); j += 3;

		}

		var geometry = new BufferGeometry();
		geometry.setAttribute( 'position', new Float32BufferAttribute( vertices, 3 ) );
		geometry.setAttribute( 'color', new Float32BufferAttribute( colors, 3 ) );

		var material = new LineBasicMaterial( { vertexColors: true, toneMapped: false } );

		super( geometry, material );

		this.type = 'GridHelper';

	}

	copy ( source ) {

		super.copy( source );

		this.geometry.copy( source.geometry );
		this.material.copy( source.material );

		return this;

	}

	clone () {

		return new GridHelper().copy( this );

	}

}

export { GridHelper };

rollup.config.js

import buble from 'rollup-plugin-buble';

...

export default [
	{
		input: 'src/Three.js',
		plugins: [
			glconstants(),
			glsl()
		],
		output: {
			format: 'umd',
			name: 'THREE',
			file: 'build/three.js',
			indent: '\t'
		}
	},
	{
		input: 'src/Three.js',
		plugins: [
			glconstants(),
			glsl()
		],
		output: [
			{
				format: 'esm',
				file: 'build/three.module.js',
				indent: '\t'
			}
		]
	},
	{
		input: 'src/Three.js',
		plugins: [
			glconstants(),
			glsl(),
			buble( {
				transforms: {
					arrow: true,
					classes: true
					// other transforms possible too
				}
			} )
		],
		output: {
			format: 'umd',
			name: 'THREE',
			file: 'build/three.legacy.js',
			indent: '\t'
		}
	}
];