Define RestrictedInternalsVisibleToAttribute

See dotnet/roslyn-analyzers#2498

eng/targets/Imports.targets

@@ -228,6 +228,29 @@
     <None Include="$(MSBuildThisFileDirectory)\..\..\src\Setup\PowerShell\uninstall.ps1" PackagePath="tools\uninstall.ps1" Visible="false" Pack="true"/>
   </ItemGroup>
 
+  <!-- RestrictedInternalsVisibleTo -->
+  <ItemDefinitionGroup>
+    <RestrictedInternalsVisibleTo>
+      <Visible>false</Visible>
+    </RestrictedInternalsVisibleTo>
+  </ItemDefinitionGroup>
+
+  <ItemGroup>
+    <InternalsVisibleTo Include="@(RestrictedInternalsVisibleTo)" Key="%(Key)" />
+  </ItemGroup>
+
+  <Target Name="PrepareGenerateRestrictedInternalsVisibleTo"
+          BeforeTargets="PrepareGenerateInternalsVisibleToFile"
+          Condition="'@(RestrictedInternalsVisibleTo)' != ''">
+    <ItemGroup>
+      <_InternalsVisibleToAttribute Include="System.Runtime.CompilerServices.RestrictedInternalsVisibleToAttribute">
+        <_Parameter1>%(RestrictedInternalsVisibleTo.Identity)</_Parameter1>
+        <_Parameter2 Condition="'%(RestrictedInternalsVisibleTo.Partner)' != ''">Microsoft.CodeAnalysis.ExternalAccess.%(RestrictedInternalsVisibleTo.Partner)</_Parameter2>
+        <_Parameter2 Condition="'%(RestrictedInternalsVisibleTo.Partner)' == ''">Microsoft.CodeAnalysis.ExternalAccess.UnknownPartner</_Parameter2>
+      </_InternalsVisibleToAttribute>
+    </ItemGroup>
+  </Target>
+
   <!--
     Ensure TargetFrameworkMonikerAssemblyAttributeText is treated as auto-generated
   -->

src/Workspaces/Core/Portable/Utilities/RestrictedInternalsVisibleToAttribute.cs

+// Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
+
+using System.Collections.Immutable;
+
+namespace System.Runtime.CompilerServices
+{
+    [AttributeUsage(AttributeTargets.Assembly, AllowMultiple = true)]
+    internal sealed class RestrictedInternalsVisibleToAttribute : Attribute
+    {
+        public RestrictedInternalsVisibleToAttribute(string assemblyName, params string[] allowedNamespaces)
+        {
+            AssemblyName = assemblyName;
+            AllowedNamespaces = allowedNamespaces.ToImmutableArray();
+        }
+
+        public string AssemblyName { get; }
+        public ImmutableArray<string> AllowedNamespaces { get; }
+    }
+}