Build "Error: Octal literal in strict mode" when css with content:"\123"

In a library created with this generator (Thanks! It's awesome) I use a css like this:
```
.css-class:before {
content: "\62";
}
```
Building with `npm run build` I have an error:
```
Starting 'rollup:fesm'...

events.js:160
throw er; // Unhandled 'error' event
^
Error: Octal literal in strict mode
at error (C:\Microarea\fabriziodebortoli\ng-icon\node_modules\rollup\dist\rollup.js:170:12)
at Module.error$1 [as error] (C:\Microarea\fabriziodebortoli\ng-icon\node_modules\rollup\dist\rollup.js:8007:2)
at tryParse (C:\Microarea\fabriziodebortoli\ng-icon\node_modules\rollup\dist\rollup.js:7710:10)
at new Module (C:\Microarea\fabriziodebortoli\ng-icon\node_modules\rollup\dist\rollup.js:7745:14)
at C:\Microarea\fabriziodebortoli\ng-icon\node_modules\rollup\dist\rollup.js:9495:17
at process._tickCallback (internal/process/next_tick.js:109:7)
```

What can I try to do?

This is an example of library: https://github.com/fabriziodebortoli/ng-icon

Reproduce the failure with npm run build in the linked example library and inspect the Rollup FESM build path for the CSS containing content: "\\62". Done means the example library builds without the strict-mode octal error; the issue names no source file or test to start from.

Written by the indexing model from the issue text.