[object CustomEvent] thrown randomly/intermittently for any of tests in angular2 test suite

### Expected behaviour
Tests should not fail intermittently without any logic issue.

### Actual behaviour

### Environment Details
Running tests on PhantomJS browser.

- Karma version (output of `karma --version`): 1.7.0
- Relevant part of your `karma.config.js` file

```
module.exports = function (config) {
config.set({
basePath: '',
frameworks: ['jasmine', '@angular/cli'],
plugins: [
require('karma-jasmine'),
require('karma-chrome-launcher'),
require('karma-jasmine-html-reporter'),
require('karma-coverage-istanbul-reporter'),
require('@angular/cli/plugins/karma'),
require('karma-junit-reporter'),
require('karma-phantomjs-launcher')
],
client:{
clearContext: false
},
coverageIstanbulReporter: {
reports: [ 'html', 'lcovonly' ],
fixWebpackSourcePaths: true,
thresholds: {
statements: 40,
lines: 40,
branches: 40,
functions: 30
}
},
angularCli: {
environment: 'dev'
},
reporters: config.angularCli && config.angularCli.codeCoverage
? ['progress','coverage-istanbul','kjhtml', 'dots', 'junit']
: ['progress', 'kjhtml', 'dots', 'junit'],
junitReporter: {
},
port: 9876,
colors: true,
logLevel: config.LOG_INFO,
autoWatch: true,
browsers: ['PhantomJS'],
singleRun: true
});
};
```

### Steps to reproduce the behaviour

1. Run a angular2 test suite with more than 50 tests in it.
2. Randomly few of tests fails with error message as '[object CustomEvent] thrown'; No further error information is present.
3.

Start with the provided karma.config.js and reproduce the failure by running the Angular test suite in PhantomJS with more than 50 tests. Investigate why intermittent failures surface only as '[object CustomEvent] thrown'; done means the tests run reliably and failures provide actionable error information.

Written by the indexing model from the issue text.