Update boot-client.ts to handle IE 11 shim issues

[ADefWebserver]

In IE 11, when you get error like this:
Unable to get property 'apply' of undefined or null reference

In functions like this:

function combine(options) {
    return ((Object)).assign.apply(((Object)), [{}].concat(options));
}	

in the vendor.js file

This article tells you to use core-js because Angular already has a dependency on it:
http://stackoverflow.com/questions/41276692/angular2-ie11-unable-to-get-property-apply-of-undefined-or-null-reference

You add:
import "core-js/client/shim";

to the top of boot-client.ts

and the problem goes away.

This issue may be related:
#926

[AmrineA]

I can confirm that this change does resolve the error referenced.

[im1dermike]

This did not resolve my issue #926

[SteveSandersonMS]

Thanks for contributing this. Since the Angular template has now moved to being Angular CLI-based, we no longer have code in this repo to determine what polyfills are placed there - we take the output from Angular CLI. As such this no longer applicable, but thanks very much for getting involved!