Speed up karma runs for development

Using DefinePlugin means the block wrapped in GENERATE_COVERAGE_REPORT
gets eliminated. Previously it was always included (with the entire
app!), even if using -f option

config/karma.config.js

@@ -6,6 +6,7 @@ const argumentsParser = require('commander');
 const webpackConfig = require('./webpack.config.js');
 
 const ROOT_PATH = path.resolve(__dirname, '..');
+const GENERATE_COVERAGE_REPORT = process.env.BABEL_ENV === 'coverage' || process.env.NODE_ENV === 'coverage';
 
 function fatalError(message) {
   console.error(chalk.red(`\nError: ${message}\n`));
@@ -22,6 +23,10 @@ webpackConfig.optimization.splitChunks = false;
 // use quicker sourcemap option
 webpackConfig.devtool = 'cheap-inline-source-map';
 
+webpackConfig.plugins.push(
+  new webpack.DefinePlugin({ GENERATE_COVERAGE_REPORT }),
+);
+
 const specFilters = argumentsParser
   .option(
     '-f, --filter-spec [filter]',
@@ -118,7 +123,7 @@ module.exports = function(config) {
     };
   }
 
-  if (process.env.BABEL_ENV === 'coverage' || process.env.NODE_ENV === 'coverage') {
+  if (GENERATE_COVERAGE_REPORT) {
     karmaConfig.reporters.push('coverage-istanbul');
     karmaConfig.coverageIstanbulReporter = {
       reports: ['html', 'text-summary'],

spec/javascripts/test_bundle.js

@@ -155,8 +155,8 @@ describe('test errors', () => {
 // if we're generating coverage reports, make sure to include all files so
 // that we can catch files with 0% coverage
 // see: https://github.com/deepsweet/istanbul-instrumenter-loader/issues/15
-if (process.env.BABEL_ENV === 'coverage') {
   // exempt these files from the coverage report
+if (GENERATE_COVERAGE_REPORT) {
   const troubleMakers = [
     './blob_edit/blob_bundle.js',
     './boards/components/modal/empty_state.vue',