What does the error "React is running in production mode, but dead code elimination has not been applied" mean?

Question

React

What might be causing this error?
On the local everything works, but when I send it to the build, then after a while the following error occurs on the build

Webpack prod -

const path = require('path');
const webpack = require('webpack');
const UglifyJSPlugin = require('uglifyjs-webpack-plugin');

module.exports = {
  entry: [
    './dev/js/index'
  ],
  output: {
    path: path.join(__dirname, 'dist/js'),
    publicPath: "/js/",
    filename: 'bundle.js',
  },
  target: 'node',
  module: {
    loaders: [
      {
        test: /\.(js|jsx)$/,
        include: path.join(__dirname, 'dev/js'),
        exclude: /node_modules/,
        loader: "babel-loader",
        options: {
          presets: [ 'react', 'es2015', 'stage-0']
        }
      },
      {
        test: /\.(sass|scss)/,
        include: path.join(__dirname, 'dev/sass'),
        loader: 'style-loader!css-loader!sass-loader'
      },
      {
        test: /\.css$/,
        loader: 'style-loader!css-loader'
      },
      {
        test: /\.(ttf|eot|svg|woff(2)?)(\?[a-z0-9=&.]+)?$/,
        loaders: ['url-loader']
      },
      {
        test: /\.(png|jpg)$/,
        loader: 'url-loader'
      }
    ]
  },

  plugins: [
    new webpack.optimize.OccurrenceOrderPlugin(),
    new webpack.DefinePlugin({
      'process.env': {
        'NODE_ENV': JSON.stringify('production')
      }
    }),
    new UglifyJSPlugin({
      "mangle": {
        "screw_ie8": true
      },
      "compress": {
        "screw_ie8": true,
        "warnings": false
      },
      "sourceMap": false
    })
  ],
};

Webpack prod-dev

const path = require('path');
const webpack = require('webpack');
const UglifyJSPlugin = require('uglifyjs-webpack-plugin');

module.exports = {
  entry: [
    './dev/js/index'
  ],
  output: {
    path: path.join(__dirname, 'dist/js'),
    publicPath: "/js/",
    filename: 'bundle.js',
  },
  target: 'node',
  module: {
    loaders: [
      {
        test: /\.(js|jsx)$/,
        include: path.join(__dirname, 'dev/js'),
        exclude: /node_modules/,
        loader: "babel-loader",
        options: {
          presets: [ 'react', 'es2015', 'stage-0']
        }
      },
      {
        test: /\.(sass|scss)/,
        include: path.join(__dirname, 'dev/sass'),
        loader: 'style-loader!css-loader!sass-loader'
      },
      {
        test: /\.css$/,
        loader: 'style-loader!css-loader'
      },
      {
        test: /\.(ttf|eot|svg|woff(2)?)(\?[a-z0-9=&.]+)?$/,
        loaders: ['url-loader']
      },
      {
        test: /\.(png|jpg)$/,
        loader: 'url-loader'
      }
    ]
  },

  plugins: [
    new webpack.optimize.OccurrenceOrderPlugin(),
    new webpack.DefinePlugin({
      'process.env': {
        'NODE_ENV': JSON.stringify('production'),
        'NODE_CUSTOM_MODE': JSON.stringify('development'),
      }
    }),
    new UglifyJSPlugin({
      "mangle": {
        "screw_ie8": true
      },
      "compress": {
        "screw_ie8": true,
        "warnings": false
      },
      "sourceMap": false
    })
  ],
};

Answer 1

At the end of the text of the error is a link with the answer to your question.
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