Add error-tracker.js to your webpack config

webpack.config.js

var path = require('path');
var webpack = require('webpack');
var WebpackStrip = require('webpack-strip');
var ExtractTextPlugin = require('extract-text-webpack-plugin');
var HtmlWebpackPlugin = require('html-webpack-plugin');

const ENV = process.env.NODE_ENV || 'development';

var config = {
  context: path.join(__dirname),
  entry: {
    app: [
      './src/index.js',
      './src/error-tracker.js',
      'node_modules/normalize.css/normalize.css',
      'node_modules/@blueprintjs/core/dist/blueprint.css',
      'node_modules/@blueprintjs/datetime/dist/blueprint-datetime.css',
      'node_modules/react-select/dist/react-select.css',
      'global_styles/_main.scss'
    ]
  }
}