Fix broken target

src/Discord.Net.Analyzers/Discord.Net.Analyzers.csproj

@@ -1,10 +1,10 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
   <Import Project="../../Discord.Net.targets" />
   <PropertyGroup>
     <AssemblyName>Discord.Net.Analyzers</AssemblyName>
     <RootNamespace>Discord.Analyzers</RootNamespace>
     <Description>A Discord.Net extension adding support for design-time analysis of the API usage.</Description>
-    <TargetFramework>netstandard2.0;netstandard2.1</TargetFramework>
+    <TargetFrameworks>netstandard2.0;netstandard2.1</TargetFrameworks>
   </PropertyGroup>
   <ItemGroup>
     <PackageReference Include="Microsoft.CodeAnalysis" Version="3.3.1" />

test/Discord.Net.Analyzers.Tests/Discord.Net.Analyzers.Tests.csproj

@@ -1,7 +1,7 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netcoreapp2.1</TargetFramework>
+    <TargetFramework>netcoreapp3.0</TargetFramework>
 
     <IsPackable>false</IsPackable>
   </PropertyGroup>

test/Discord.Net.Analyzers.Tests/Extensions/AppDomainPolyfill.cs

@@ -1,30 +0,0 @@
-using System.Linq;
-using System.Reflection;
-using Microsoft.DotNet.PlatformAbstractions;
-using Microsoft.Extensions.DependencyModel;
-
-namespace System
-{
-    /// <summary> Polyfill of the AppDomain class from full framework. </summary>
-    internal class AppDomain
-    {
-        public static AppDomain CurrentDomain { get; private set; }
-
-        private AppDomain()
-        {
-        }
-
-        static AppDomain()
-        {
-            CurrentDomain = new AppDomain();
-        }
-
-        public Assembly[] GetAssemblies()
-        {
-            var rid = RuntimeEnvironment.GetRuntimeIdentifier();
-            var ass = DependencyContext.Default.GetRuntimeAssemblyNames(rid);
-
-            return ass.Select(Assembly.Load).ToArray();
-        }
-    }
-}