move this.requiredtabindex = -1 into ngAfterViewInit, due to two reasons:
a. before change: In the context of requiresExplicitTabIndex execution, tag does not have href attribute populated yet. For detail see this discussion thread
as a result, we have to put it to ngOnAfterViewInit, at that time the href attribute of a tag is already populated with correct value：
b. The problem here is that we have a situation where the view generation process (which ngAfterViewInit is a part of) is itself further modifying the data that we are trying to display in the first place.
In unit test code it reports error message:
See this blog.
So I use setTimeout() to defer the code to another Javascript Virtual Machine turn.
Since now this.requiredtabindex = -1 is wrapped to setTimeout and executed asynchronously, some unit test will fail: