Missing error handler on `socket` when I get "Delaying execution, these browsers are not ready: Chrome" from previous test

### Expected behaviour
I expect Karma not to crash when it runs the test again after displaying "Delaying execution, these browsers are not ready: Chrome" from the previous test.

### Actual behaviour
This happens when I make some changes and then save my file. Before karma finishes running, if I do another save after a change, it will first interrupt the test with `04 01 2017 15:00:37.775:INFO [karma]: Delaying execution, these browsers are not ready: Chrome 55.0.2883 (Mac OS X 10.11.6)`.

Then, it will finish the rest of the expectations (they are all green), and then display

```
=========================================================================================================================

START:

Finished in 0 secs / 0 secs
```

before running the test again, because of that second save that I did. Then it will run through all my tests again (all green), and then give this error with the missing error handler on socket:

```
Missing error handler on `socket`.
TypeError: Cannot read property 'set' of null
at KarmaRemapIstanbul.onBrowserComplete (/Users/kamok/code/election/election-mobile/node_modules/karma-remap-istanbul/index.js:28:16)
at Server. (/Users/kamok/code/election/election-mobile/node_modules/karma/lib/events.js:13:22)
at emitTwo (events.js:111:20)
at Server.emit (events.js:191:7)
at Browser.onComplete (/Users/kamok/code/election/election-mobile/node_modules/karma/lib/browser.js:143:13)
at Socket. (/Users/kamok/code/election/election-mobile/node_modules/karma/lib/events.js:13:22)
at emitTwo (events.js:111:20)
at Socket.emit (events.js:191:7)
at Socket.onevent (/Users/kamok/code/election/election-mobile/node_modules/karma/node_modules/socket.io/lib/socket.js:335:8)
at Socket.onpacket (/Users/kamok/code/election/election-mobile/node_modules/karma/node_modules/socket.io/lib/socket.js:295:12)
at Client.ondecoded (/Users/kamok/code/election/election-mobile/node_modules/karma/node_modules/socket.io/lib/client.js:193:14)
at Decoder.Emitter.emit (/Users/kamok/code/election/election-mobile/node_modules/component-emitter/index.js:134:20)
at Decoder.add (/Users/kamok/code/election/election-mobile/node_modules/socket.io-parser/index.js:247:12)
at Client.ondata (/Users/kamok/code/election/election-mobile/node_modules/karma/node_modules/socket.io/lib/client.js:175:18)
at emitOne (events.js:96:13)
at Socket.emit (events.js:188:7)
at Socket.onPacket (/Users/kamok/code/election/election-mobile/node_modules/karma/node_modules/engine.io/lib/socket.js:101:14)
at emitOne (events.js:96:13)
at WebSocket.emit (events.js:188:7)
at WebSocket.Transport.onPacket (/Users/kamok/code/election/election-mobile/node_modules/karma/node_modules/engine.io/lib/transport.js:104:8)
at WebSocket.Transport.onData (/Users/kamok/code/election/election-mobile/node_modules/karma/node_modules/engine.io/lib/transport.js:115:8)
at WebSocket.onData (/Users/kamok/code/election/election-mobile/node_modules/karma/node_modules/engine.io/lib/transports/websocket.js:76:30)
at emitTwo (events.js:106:13)
at WebSocket.emit (events.js:191:7)
at Receiver.ontext (/Users/kamok/code/election/election-mobile/node_modules/karma/node_modules/ws/lib/WebSocket.js:816:10)
at /Users/kamok/code/election/election-mobile/node_modules/karma/node_modules/ws/lib/Receiver.js:477:18
at /Users/kamok/code/election/election-mobile/node_modules/karma/node_modules/ws/lib/Receiver.js:361:7
at /Users/kamok/code/election/election-mobile/node_modules/karma/node_modules/ws/lib/PerMessageDeflate.js:247:5
at afterWrite (_stream_writable.js:387:3)
at onwrite (_stream_writable.js:378:7)
at WritableState.onwrite (_stream_writable.js:89:5)
at afterTransform (_stream_transform.js:79:3)
at TransformState.afterTransform (_stream_transform.js:54:12)
at Zlib.callback (zlib.js:639:5)
04 01 2017 15:00:48.724:WARN [Chrome 55.0.2883 (Mac OS X 10.11.6)]: Disconnected (1 times)
Chrome 55.0.2883 (Mac OS X 10.11.6) ERROR
Disconnectedundefined
Chrome 55.0.2883 (Mac OS X 10.11.6) ERROR
Disconnectedundefined
```

### Environment Details

- Karma version (output of `karma --version`):
Karma version: 1.3.0
- Relevant part of your `karma.config.js` file
```
basePath: '',
frameworks: [
'jasmine',
'angular-cli',
'jasmine-matchers' // https://github.com/JamieMason/karma-jasmine-matchers
],
plugins: [
require('karma-phantomjs-launcher'),
require('karma-jasmine'),
require('karma-chrome-launcher'),
require('karma-remap-istanbul'),
require('karma-mocha-reporter'),
require('angular-cli/plugins/karma'),
require('karma-jasmine-matchers') // https://github.com/JamieMason/karma-jasmine-matchers
],
files: [ // http://karma-runner.github.io/1.0/config/files.html
{ pattern: './src/test.ts', watched: false },
{ pattern: './www/assets/images/*.jpg', watched: false, included: false, served: true},
{ pattern: './www/json/*.json', watched: false, included: false, served: true}
],
preprocessors: {
'./src/test.ts': ['angular-cli']
},
remapIstanbulReporter: {
reports: {
html: 'coverage',
lcovonly: './coverage/coverage.lcov'
}
},
proxies: {
'/assets/': '/base/www/assets/',
'/json/': '/base/www/json/',
},
angularCli: {
config: './angular-cli.json',
environment: 'dev'
},
reporters: [
'mocha', 'karma-remap-istanbul'
],
port: 9876,
colors: true,
logLevel: config.LOG_INFO,
autoWatch: true,
browsers: ['Chrome'], // 'PhantomJS' or 'Chrome'
singleRun: false,
mime: { 'text/x-typescript': ['ts', 'tsx'] }
```
### Steps to reproduce the behaviour

See above. This is only is Chrome. On PhantomJS it works fine. Basically Karma shutsdown if I save again before the first test is finished running.

Reproduce the Chrome failure by saving again while the previous Karma run is still finishing. Start with karma-remap-istanbul/index.js at onBrowserComplete and trace the Karma flow through karma/lib/events.js and karma/lib/browser.js. Done means repeated saves no longer cause the missing socket error, crash, or Chrome disconnection.

Written by the indexing model from the issue text.