Karma coverage empty with a simple sample

sample project:
https://github.com/njleonzhang/karma-study/tree/master/karma-2

Here is the config file:

```
// Karma configuration

module.exports = function(config) {
config.set({
// plugins
plugins: [
'karma-phantomjs-launcher',
'karma-coverage'
],

// list of files / patterns to load in the browser
files: [
'src/index.js',
'test/index.js'
],

reporters: ['coverage'],

// preprocess matching files before serving them to the browser
// available preprocessors: https://npmjs.org/browse/keyword/karma-preprocessor
preprocessors: {
'src/index.js': ['coverage']
},

coverageReporter: {
dir: './coverage',
// includeAllSources: true,
reporters: [
{ type: 'lcov', subdir: '.' },
{ type: 'text-summary' }
]
},

// enable / disable colors in the output (reporters and logs)
colors: true,
.....
}
```

result:
```
=============================== Coverage summary===============================
Statements : 100% ( 0/0 )
Branches : 100% ( 0/0 )
Functions : 100% ( 0/0 )
Lines : 100% ( 0/0 )
================================================================================

```

version:
"karma": "^1.7.1",
"karma-coverage": "^1.1.1",

Reproduce the report using the linked karma-2 sample project and its Karma configuration, focusing on src/index.js, test/index.js, and the coverage preprocessor settings. Compare the generated coverage output with the sample's execution, then verify that the coverage summary reports the expected source files and non-zero totals.

Written by the indexing model from the issue text.