Angular 2 testing - process.env

I try to mock requests in my application, but there is a problem with process variable. I store in process.env.backendUrl url to backend API. And then in RestService I have:

constructor(private http: Http) {
    this.rest = process.env.backendUrl + "/api/";
}

And now it is impossible to run tests because in for example LoginComponent I have RestService dependency and I've got this error:

zone.js:140 Uncaught Error: Error in ./LoginComponent class LoginComponent_Host - inline template:0:0 caused by: process is not defined
ReferenceError: process is not defined
at new RestService (http://localhost:9876/base/src/test.ts:21595:2790)
at DynamicTestModuleInjector.get (DynamicTestModule.ngfactory.js:170:67)
at DynamicTestModuleInjector.get (DynamicTestModule.ngfactory.js:180:93)
at DynamicTestModuleInjector.getInternal (DynamicTestModule.ngfactory.js:255:51)
at DynamicTestModuleInjector.NgModuleInjector.get (http://localhost:9876/base/src/test.ts:25036:27)
at TestBed.get (http://localhost:9876/base/src/test.ts:5589:51)
at _View_LoginComponent_Host0.createInternal (LoginComponent_Host.ngfactory.js:16:74)
at _View_LoginComponent_Host0.AppView.create (http://localhost:9876/base/src/test.ts:36192:21)
at _View_LoginComponent_Host0.DebugAppView.create (http://localhost:9876/base/src/test.ts:36404:44)

I set proccess.env.backendUrl in enviroment.ts (file created by angular-cli).

process.env.backendUrl = 'http://localhost:8080';

export const environment = {
  production: false
};

Should I set it somewhere else or is there any method to tell karma about this variable?