Regression: JSON reporters are broken after Karma 2.0.1

[cexbrayat]

Starting from karma 2.0.1, the JSON reporters available:

• https://www.npmjs.com/package/karma-json-reporter
• https://www.npmjs.com/package/karma-structured-json-reporter
• https://www.npmjs.com/package/karma-json-result-reporter

are all failing as Karma now returns an object with a circular reference and all the reporters use JSON.stringify to generate their reports.

Expected behaviour

The JSON reporters should continue to work as they were previously.

Actual behaviour

Running a simple test with karma-json-reporter produces:

Chrome 67.0.3396 (Mac OS X 10.13.5): Executed 43 of 43 SUCCESS (9.014 secs / 8.844 secs)
10 07 2018 11:30:43.905:ERROR [karma]: TypeError: Converting circular structure to JSON
    at JSON.stringify (<anonymous>)
    at /Users/cedric/Code/ninjasquad/ponyracer/node_modules/karma-json-reporter/index.js:35:24
    at /Users/cedric/Code/ninjasquad/ponyracer/node_modules/karma/lib/helper.js:145:7
    at /Users/cedric/Code/ninjasquad/ponyracer/node_modules/graceful-fs/polyfills.js:287:18
    at FSReqWrap.oncomplete (fs.js:153:5)

Environment Details

• Karma version (output of karma --version): 2.0.2 (same issue with 2.0.3 and 2.0.4)
• Relevant part of your karma.config.js file

module.exports = function (config) {
  config.set({
    basePath: '',
    browserNoActivityTimeout: 30000,
    frameworks: ['jasmine', '@angular-devkit/build-angular'],
    plugins: [
      require('karma-jasmine'),
      require('karma-chrome-launcher'),
      require('karma-jasmine-html-reporter'),
      require('karma-coverage-istanbul-reporter'),
      require('karma-firefox-launcher'),
      require('karma-json-reporter'),
      require('@angular-devkit/build-angular/plugins/karma')
    ],
    client: {
      clearContext: false // leave Jasmine Spec Runner output visible in browser
    },
    coverageIstanbulReporter: {
      dir: require('path').join(__dirname, '../coverage'),
      reports: [ 'html', 'json-summary'],
      fixWebpackSourcePaths: true
    },
    jsonReporter: {
      stdout: false,
      outputFile: '../results/karma-results.json'
    },
    reporters: ['progress', 'json', 'kjhtml'],
    port: 9876,
    colors: true,
    logLevel: config.LOG_INFO,
    autoWatch: true,
    browsers: ['Chrome'],
    singleRun: false
  });
};

Steps to reproduce the behaviour

1. Add karma-json-reporter to karma.config.js
2. Run the tests

Other info

After a brief analysis, it looks like the migration of the Browser class to ES2015 introduced the regression (see bb012e2) as it added a bunch of fields on the Browser object, some of them self referencing.

[cexbrayat]

I submitted #3076 as a potential fix for this issue, by adding a toJSON() method to the problematic Browser class.