fix(karma-webpack): normalize paths (windows)

[alabbas-ali]

This error show on windows OS:

04 09 2018 16:02:38.163:ERROR [karma]: TypeError: Path must be a string. Received undefined
  at assertPath (path.js:28:11)
  at Object.join (path.js:499:7)
  at Plugin.<anonymous> (..\node_modules\karma-webpack\lib\karma-webpack.js:262:68)
  at Plugin.readFile (..\node_modules\karma-webpack\lib\karma-webpack.js:281:5)
  at _combinedTickCallback (internal/process/next_tick.js:131:7)
  at process._tickCallback (internal/process/next_tick.js:180:9)

04 09 2018 16:02:38.178:ERROR [karma]: TypeError: Path must be a string. Received undefined
  at assertPath (path.js:28:11)
  at Object.join (path.js:499:7)
  at Plugin.<anonymous> (..\node_modules\karma-webpack\lib\karma-webpack.js:262:68)
  at Plugin.readFile (..\node_modules\karma-webpack\lib\karma-webpack.js:281:5)
  at _combinedTickCallback (internal/process/next_tick.js:131:7)
  at Immediate._tickCallback [as _onImmediate] (internal/process/next_tick.js:180:9)
  at runCallback (timers.js:789:20)
  at tryOnImmediate (timers.js:751:5)
  at processImmediate [as _immediateCallback] (timers.js:722:5)

The output assets array has indexes like 'path/to/file' and the file name is 'path\to\file'

Type

• Fix

Issues

• Fixes TypeError: Path must be a string. Received [ 'js/main.spec.js', #350

SemVer

• Patch

[jsf-clabot]

All committers have signed the CLA.

[michael-ciniawsky]

@alabbas-ali Thx

[michael-ciniawsky]

Released in v3.0.3 🎉