It looks like the igx-form-group__input--search class requires another stylesheet imported somewhere, but I'm not finding a reference to make it work in like in stackblitz
Mine
https://github.com/djbreen7/IgniteUISandBox
https://djbreen7.github.io/IgniteUISandBox/
Infragistics Demo
https://stackblitz.com/angular/pernmgqeavg
I'm sure it's a simple fix. I just can't see what I'm missing.
It seems like the class names you mean are valid for igniteui-angular 5.2.1 version, while in the version of your Sandbox app (igniteui-angular 5.3.0-rc) those have been changed.
Please note that this latter version introduces Input Group, which makes creating forms even easier: https://www.infragistics.com/products/ignite-ui-angular/angular/components/input-group