I would love to comment. But due to missing reputation I write an answer. Well I know two possibilities to solve this issue.

1. Inject the Global Document. Well it might not be the best practice as I don't know if nativescript etc supports that. But it is at least better than use pure JS.

constructor(@Inject(DOCUMENT) private document: Document) {}

ngOnInit(){
   this.document.body.classList.add('test');
}

Well and perhaps even better. You could inject the renderer or renderer 2 (on NG4) and add the class with the renderer.

export class myModalComponent implements OnDestroy {

  constructor(private renderer: Renderer) {
    this.renderer.setElementClass(document.body, 'modal-open', true);
  }

  ngOnDestroy(): void {
    this.renderer.setElementClass(document.body, 'modal-open', false);
  }
}

EDIT FOR ANGULAR4:

import { Component, OnDestroy, Renderer2 } from '@angular/core';

export class myModalComponent implements OnDestroy {

  constructor(private renderer: Renderer2) {
    this.renderer.addClass(document.body, 'modal-open');
  }

  ngOnDestroy(): void {
    this.renderer.removeClass(document.body, 'modal-open');
  }
}

I think the best way to do it is a combination of both approaches by DaniS above: Using the renderer to actually set / remove the class, but also using the document injector, so it is not strongly dependant on the window.document but that can be replaced dynamically (e.g. when rendering server-side). So the whole code would look like this:

import { DOCUMENT } from '@angular/common';
import { Component, Inject, OnDestroy, OnInit, Renderer2 } from '@angular/core';

@Component({ /* ... */ })
export class MyModalComponent implements OnInit, OnDestroy {
    constructor (
        @Inject(DOCUMENT) private document: Document,
        private renderer: Renderer2,
    ) { }

    ngOnInit(): void {
        this.renderer.addClass(this.document.body, 'embedded-body');
    }

    ngOnDestroy(): void {
        this.renderer.removeClass(this.document.body, 'embedded-body');
    }
}