To cover this particular function one needs to call app.configHttpClient() either directly, or by calling app.activate().
The httpClient is aurelia's fetch wrapper (docs: http://aurelia.io/hub.html#/doc/article/aurelia/fetch-client/latest/http-services/2) which is initialized in the App's constructor.
By default it's @injected by aurelia
…
import {inject} from 'aurelia-framework';
import {HttpClient} from 'aurelia-fetch-client';