Style Classes don't play with Resource Keys

Description

Resource Keys override style classes.

Steps to Reproduce

1. MainPage.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="XFBindingTest.MainPage">
    <ContentPage.Resources>
        <ResourceDictionary>
            <Style TargetType="BoxView" Class="Base">
                <Setter Property="BackgroundColor" Value="Lime"/>
            </Style>
        </ResourceDictionary>
    </ContentPage.Resources>

    <BoxView StyleClass="Base"/>

</ContentPage>

2. Run
3. See lime color

-            <Style TargetType="BoxView" Class="Base">
+            <Style TargetType="BoxView" Class="Base" x:Key="Key">

5. Run
6. See white color

Expected Behavior

6. See lime color

Actual Behavior

6. See white color

Basic Information

• Version with issue: 3.4.0.1008975
• IDE: Visual Studio 2017
• Platform Target Frameworks:
  • UWP: 16299

Screenshots

6.

Related

#4895