Use GitLab UI toast in global_toast

app/assets/javascripts/vue_shared/plugins/global_toast.js

@@ -2,7 +2,8 @@ import Vue from 'vue';
 import { GlToast } from '@gitlab/ui';
 
 Vue.use(GlToast);
+const instance = new Vue();
 
 export default function showGlobalToast(...args) {
-  return Vue.toasted.show(...args);
+  return instance.$toast.show(...args);
 }

spec/frontend/vue_shared/plugins/global_toast_spec.js

-import toast from '~/vue_shared/plugins/global_toast';
 import Vue from 'vue';
+import toast from '~/vue_shared/plugins/global_toast';
 
 describe('Global toast', () => {
   let spyFunc;
 
   beforeEach(() => {
-    spyFunc = jest.spyOn(Vue.toasted, 'show').mockImplementation(() => {});
+    spyFunc = jest.spyOn(Vue.prototype.$toast, 'show').mockImplementation(() => {});
   });
 
   afterEach(() => {
     spyFunc.mockRestore();
   });
 
-  it('should pass all args to Vue toasted', () => {
+  it("should call GitLab UI's toast method", () => {
     const arg1 = 'TestMessage';
     const arg2 = { className: 'foo' };
 
     toast(arg1, arg2);
 
-    expect(Vue.toasted.show).toHaveBeenCalledTimes(1);
-    expect(Vue.toasted.show).toHaveBeenCalledWith(arg1, arg2);
+    expect(Vue.prototype.$toast.show).toHaveBeenCalledTimes(1);
+    expect(Vue.prototype.$toast.show).toHaveBeenCalledWith(arg1, arg2);
   });
 });