Use inline source maps for karma (!11815)

config/karma.config.js

@@ -13,6 +13,8 @@ if (webpackConfig.plugins) {
   });
 }
 
+webpackConfig.devtool = 'cheap-inline-source-map';
+
 // Karma configuration
 module.exports = function(config) {
   var progressReporter = process.env.CI ? 'mocha' : 'progress';

config/webpack.config.js

@@ -72,8 +72,6 @@ var config = {
     filename: IS_PRODUCTION ? '[name].[chunkhash].bundle.js' : '[name].bundle.js'
   },
 
-  devtool: 'cheap-module-source-map',
-
   module: {
     rules: [
       {