using included Symbol Icons for AppBar with winrt

How do I make use of the symbol icons included with WINRT?

So usually in winrt I just do this

<Page.BottomAppBar>
        <CommandBar>
            <CommandBar.PrimaryCommands>
                <AppBarButton Icon="Previous"

And I get a previous arrow from the set of default glyphs included with winrt

But doing this in Xamarin Forms

<ToolbarItem  Name="Previous" Icon="Previous">
    </ToolbarItem>

I just get an X.... How do I make it so on WINRT it uses the symbol icon for Previous? I started going down the path of using a custom renderer on the page to access the appbar but ran into a few issues so figured I'd ask if there's just something silly I'm doing incorrectly