old_contents: var path = require( 'path' );
  var babel = require( 'babel-core' );
  var createFilter = require( 'rollup-pluginutils' ).createFilter;
  var assign = require( 'object-assign' );
  
  module.exports = function ( options ) {
  	options = assign( {}, options || {} );
  	var usedHelpers = [];
  	var index;
  
  	var filter = createFilter( options.include, options.exclude );
  	delete options.include;
  	delete options.exclude;
  
  	// preflight check
  	var check = babel.transform( 'export default class Foo {}', assign({ filename: path.resolve( 'test.js' ) }, options ) ).code;
  	if ( ~check.indexOf( 'function _classCallCheck' ) ) throw new Error( 'External helpers are not enabled. Please add the "external-helpers-2" plugin or use the "es2015-rollup" preset. See https://github.com/rollup/rollup-plugin-babel#TK for more information' );
  	if ( !~check.indexOf( 'export default' ) ) throw new Error( 'It looks like your Babel configuration specifies a module transformer. Please disable it. If you\'re using the "es2015" preset, consider using "es2015-rollup" instead. See https://github.com/rollup/rollup-plugin-babel#TK for more information' );
  
  	if ( options.sourceMap !== false ) options.sourceMaps = true;
  	if ( options.sourceMaps !== false ) options.sourceMaps = true;
  	delete options.sourceMap;
  
  	return {
  		transform: function ( code, id ) {
  			if ( !filter( id ) ) return null;
  
  			var transformed = babel.transform( code, assign({ filename: id }, options ) );
  
  			transformed.metadata.usedHelpers.forEach( function ( helper ) {
  				if ( !~usedHelpers.indexOf( helper ) ) usedHelpers.push( helper );
  			});
  
  			return {
  				code: transformed.code,
  				map: transformed.map
  			};
  		},
  		intro: function () {
  			// TODO replace babelHelpers.foo with babelHelpers_foo – though first
  			// we need the ability to find and replace within the generated bundle
  			return usedHelpers.length ?
  				babel.buildExternalHelpers( usedHelpers, 'var' ).trim() :
  				'';
  		}
  	};
  };
  
new_contents: var path = require( 'path' );
  var babel = require( 'babel-core' );
  var createFilter = require( 'rollup-pluginutils' ).createFilter;
  var assign = require( 'object-assign' );
  
  module.exports = function ( options ) {
  	options = assign( {}, options || {} );
  	var usedHelpers = [];
  
  	var filter = createFilter( options.include, options.exclude );
  	delete options.include;
  	delete options.exclude;
  
  	// preflight check
  	var check = babel.transform( 'export default class Foo {}', assign({ filename: path.resolve( 'test.js' ) }, options ) ).code;
  	if ( ~check.indexOf( 'function _classCallCheck' ) ) throw new Error( 'External helpers are not enabled. Please add the "external-helpers-2" plugin or use the "es2015-rollup" preset. See https://github.com/rollup/rollup-plugin-babel#TK for more information' );
  	if ( !~check.indexOf( 'export default' ) ) throw new Error( 'It looks like your Babel configuration specifies a module transformer. Please disable it. If you\'re using the "es2015" preset, consider using "es2015-rollup" instead. See https://github.com/rollup/rollup-plugin-babel#TK for more information' );
  
  	if ( options.sourceMap !== false ) options.sourceMaps = true;
  	if ( options.sourceMaps !== false ) options.sourceMaps = true;
  	delete options.sourceMap;
  
  	return {
  		transform: function ( code, id ) {
  			if ( !filter( id ) ) return null;
  
  			var transformed = babel.transform( code, assign({ filename: id }, options ) );
  
  			transformed.metadata.usedHelpers.forEach( function ( helper ) {
  				if ( !~usedHelpers.indexOf( helper ) ) usedHelpers.push( helper );
  			});
  
  			return {
  				code: transformed.code,
  				map: transformed.map
  			};
  		},
  		intro: function () {
  			// TODO replace babelHelpers.foo with babelHelpers_foo – though first
  			// we need the ability to find and replace within the generated bundle
  			return usedHelpers.length ?
  				babel.buildExternalHelpers( usedHelpers, 'var' ).trim() :
  				'';
  		}
  	};
  };
  
subject: Remove unused variable.