Use MonoMac for OSX by default

src/Avalonia.DotNetCoreRuntime/AppBuilder.cs

@@ -37,12 +37,15 @@ namespace Avalonia
         /// <returns>An <see cref="AppBuilder"/> instance.</returns>
         public AppBuilder UsePlatformDetect()
         {
+            var os = RuntimePlatform.GetRuntimeInfo().OperatingSystem;
             //We don't have the ability to load every assembly right now, so we are
             //stuck with manual configuration  here
             //Helpers are extracted to separate methods to take the advantage of the fact
             //that CLR doesn't try to load dependencies before referencing method is jitted
-            if (RuntimePlatform.GetRuntimeInfo().OperatingSystem == OperatingSystemType.WinNT)
+            if (os == OperatingSystemType.WinNT)
                 LoadWin32();
+            else if(os==OperatingSystemType.OSX)
+                LoadMonoMac();
             else
                 LoadGtk3();
             this.UseSkia();
@@ -50,6 +53,7 @@ namespace Avalonia
             return this;
         }
 
+        void LoadMonoMac() => this.UseMonoMac();
         void LoadWin32() => this.UseWin32();
         void LoadGtk3() => this.UseGtk3();
     }

src/OSX/Avalonia.MonoMac/AssemblyInfo.cs

@@ -0,0 +1,5 @@
+using System;
+using System.Collections.Generic;
+using System.Text;using Avalonia.MonoMac;
+using Avalonia.Platform;
+[assembly: ExportWindowingSubsystem(OperatingSystemType.OSX, 1, "MonoMac", typeof(MonoMacPlatform), nameof(MonoMacPlatform.Initialize))]]

src/OSX/Avalonia.MonoMac/Avalonia.MonoMac.csproj

@@ -2,8 +2,10 @@
   <PropertyGroup>
     <TargetFrameworks>netstandard2.0</TargetFrameworks>
     <AllowUnsafeBlocks>True</AllowUnsafeBlocks>
+    <GenerateAssemblyInfo>false</GenerateAssemblyInfo>
   </PropertyGroup>
   <ItemGroup>
+    <Compile Include="..\..\Shared\SharedAssemblyInfo.cs" Link="SharedAssemblyInfo.cs" />
     <Compile Include="..\..\Shared\WindowResizeDragHelper.cs" Link="WindowResizeDragHelper.cs" />
   </ItemGroup>
   <ItemGroup>