Adds FontImage Markup Extension for FontImageSource

Xamarin.Forms.Xaml.UnitTests/FontImageExtension.xaml

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+<?xml version="1.0" encoding="UTF-8"?>
+<TabBar 
+    xmlns="http://xamarin.com/schemas/2014/forms"
+    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+    xmlns:local="using:Xamarin.Forms.Xaml.UnitTests"
+    x:Class="Xamarin.Forms.Xaml.UnitTests.FontImageExtension">
+
+    <Tab Title="Hello" Icon="{FontImage Size={x:Static local:FontImageExtension.Size}, Color={x:Static local:FontImageExtension.Color}, FontFamily={x:Static local:FontImageExtension.FontFamily}, Glyph={x:Static local:FontImageExtension.Glyph}}" />
+    <Tab Title="World" Icon="{FontImage Size={x:Static local:FontImageExtension.Size}, Color={x:Static local:FontImageExtension.Color}, FontFamily={x:Static local:FontImageExtension.FontFamily}, Glyph={x:Static local:FontImageExtension.Glyph}}" />
+    <Tab Title="ContentProperty" Icon="{FontImage MyGlyph, Size={x:Static local:FontImageExtension.Size}, Color={x:Static local:FontImageExtension.Color}, FontFamily={x:Static local:FontImageExtension.FontFamily}}" />
+
+</TabBar>

Xamarin.Forms.Xaml.UnitTests/FontImageExtension.xaml.cs

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+using NUnit.Framework;
+
+using Xamarin.Forms.Core.UnitTests;
+
+namespace Xamarin.Forms.Xaml.UnitTests
+{
+	public partial class FontImageExtension : TabBar
+	{
+		public FontImageExtension() => InitializeComponent();
+		public FontImageExtension(bool useCompiledXaml)
+		{
+			//this stub will be replaced at compile time
+		}
+
+		public static string FontFamily => "MyFontFamily";
+		public static string Glyph => "MyGlyph";
+		public static Color Color => Color.Black;
+		public static double Size = 50d;
+
+		[TestFixture]
+		class Tests
+		{
+			[SetUp] public void Setup() => Device.PlatformServices = new MockPlatformServices();
+			[TearDown] public void TearDown() => Device.PlatformServices = null;
+
+			[TestCase(true), TestCase(false)]
+			public void FontImageExtension_Positive(bool useCompiledXaml)
+			{
+				var layout = new FontImageExtension(useCompiledXaml);
+				var tabs = layout.AllChildren;
+
+				foreach (var tab in tabs)
+				{
+					Tab myTab = (Tab)tab;
+					if (myTab == null)
+						continue;
+
+					Assert.That(myTab.Icon, Is.TypeOf<FontImageSource>());
+
+					var fontImage = (FontImageSource)myTab.Icon;
+					Assert.AreEqual(FontFamily, fontImage.FontFamily);
+					Assert.AreEqual(Glyph, fontImage.Glyph);
+					Assert.AreEqual(Size, fontImage.Size);
+					Assert.AreEqual(Color, fontImage.Color);
+				}
+			}
+
+			[TestCase(true), TestCase(false)]
+			public void FontImageExtension_Negative(bool useCompiledXaml)
+			{
+				var layout = new FontImageExtension(useCompiledXaml);
+				var tabs = layout.AllChildren;
+
+				foreach (var tab in tabs)
+				{
+					Tab myTab = (Tab)tab;
+					if (myTab == null)
+						continue;
+
+					Assert.That(myTab.Icon, Is.Not.TypeOf<ImageSource>());
+				}
+			}
+		}
+	}
+}

Xamarin.Forms.Xaml/MarkupExtensions/FontImageExtension.cs

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+using System;
+
+namespace Xamarin.Forms.Xaml
+{
+	[AcceptEmptyServiceProvider]
+	[ContentProperty(nameof(Glyph))]
+	public class FontImageExtension : IMarkupExtension<ImageSource>
+	{
+		public string FontFamily { get; set; }
+		public string Glyph { get; set; }
+		public Color Color { get; set; }
+		public double Size { get; set; } = (double)FontImageSource.SizeProperty.DefaultValue;
+
+		public ImageSource ProvideValue(IServiceProvider serviceProvider)
+		{
+			return new FontImageSource
+			{
+				FontFamily = FontFamily,
+				Glyph = Glyph,
+				Color = Color,
+				Size = Size
+			};
+		}
+
+		object IMarkupExtension.ProvideValue(IServiceProvider serviceProvider)
+		{
+			return (this as IMarkupExtension<ImageSource>).ProvideValue(serviceProvider);
+		}
+	}
+}