old_contents: const webpack = require('webpack');
  const ExtractTextPlugin = require('extract-text-webpack-plugin');
  const StatsPlugin = require('stats-webpack-plugin');
  const path = require('path');
  
  const featureFlagsPlugin = new webpack.DefinePlugin({
    __DEV__: false,
    __RELEASE__: true,
  });
  
  
  module.exports = {
    devtool: 'source-map',
    entry: [
      './src/index.js',
    ],
    output: {
      path: path.join(__dirname, '../static/dist'),
      filename: 'client.min.js',
      publicPath: '/',
    },
    plugins: [
      new webpack.optimize.OccurenceOrderPlugin(),
      new ExtractTextPlugin('bundle.css'),
      new webpack.optimize.UglifyJsPlugin({
        compressor: {
          warnings: true,
          screw_ie8: true,
        },
      }),
      new StatsPlugin('webpack.stats.json', {
        source: false,
        modules: false,
      }),
      new webpack.DefinePlugin({
        'process.env': {
          NODE_ENV: JSON.stringify('production'),
          BROWSER: '"true"',
        },
      }),
      featureFlagsPlugin,
      new webpack.optimize.DedupePlugin(),
    ],
    module: {
      loaders: [
        {
          test: /\.js[x]?$/,
          exclude: [
            /node_modules/,
          ],
          loader: 'babel',
          query: {
            presets: ['es2015', 'stage-0', 'react'],
            plugins: [
              ['fast-async'],
              ['transform-decorators-legacy'],
            ],
          },
        },
        {
          test: /\.json?$/,
          loader: 'json-loader',
        },
        { 
          test: /worker\.js$/,
          loader: 'worker-loader?inline=true',
        },
      ],
    },
    resolve: {
      alias: {
        react: path.join(__dirname, '../node_modules/react'),
      },
      modulesDirectories: [
        'src',
        'node_modules',
        'web_modules',
      ],
      extensions: ['', '.json', '.js', '.jsx'],
    },
    node: {
      __dirname: true,
      fs: 'empty',
    },
  };
  
new_contents: const webpack = require('webpack');
  const ExtractTextPlugin = require('extract-text-webpack-plugin');
  const StatsPlugin = require('stats-webpack-plugin');
  const path = require('path');
  
  const featureFlagsPlugin = new webpack.DefinePlugin({
    __DEV__: false,
    __RELEASE__: true,
  });
  
  
  module.exports = {
    devtool: 'cheap-module-source-map',
    entry: [
      './src/index.js',
    ],
    output: {
      path: path.join(__dirname, '../static/dist'),
      filename: 'client.min.js',
      publicPath: '/',
    },
    plugins: [
      new webpack.optimize.OccurenceOrderPlugin(),
      new ExtractTextPlugin('bundle.css'),
      new webpack.optimize.UglifyJsPlugin({
        compressor: {
          warnings: true,
          screw_ie8: true,
        },
      }),
      new StatsPlugin('webpack.stats.json', {
        source: false,
        modules: false,
      }),
      new webpack.DefinePlugin({
        'process.env': {
          NODE_ENV: JSON.stringify('production'),
          BROWSER: '"true"',
        },
      }),
      featureFlagsPlugin,
      new webpack.optimize.DedupePlugin(),
    ],
    module: {
      loaders: [
        {
          test: /\.js[x]?$/,
          exclude: [
            /node_modules/,
          ],
          loader: 'babel',
          query: {
            presets: ['es2015', 'stage-0', 'react'],
            plugins: [
              ['fast-async'],
              ['transform-decorators-legacy'],
            ],
          },
        },
        {
          test: /\.json?$/,
          loader: 'json-loader',
        },
        { 
          test: /worker\.js$/,
          loader: 'worker-loader?inline=true',
        },
      ],
    },
    resolve: {
      alias: {
        react: path.join(__dirname, '../node_modules/react'),
      },
      modulesDirectories: [
        'src',
        'node_modules',
        'web_modules',
      ],
      extensions: ['', '.json', '.js', '.jsx'],
    },
    node: {
      __dirname: true,
      fs: 'empty',
    },
  };
  
subject: use different sourcemap