Cannot statically evaluate fs argument?

While running npm run dev, the below error popped up.

Cannot statically evaluate fs argument
[0]   50 |   // Read file and split into lines
[0]   51 |   var map = {},
[0] > 52 |       content = fs.readFileSync(file, 'ascii'),
[0]      |                                ^
[0]   53 |       lines = content.split(/[\r\n]+/);
[0]   54 |
[0]   55 |   lines.forEach(function(line) {