[dotnet-linker] Add a CollectUnmarkedMembersStep that will keep linked away types around for the static registrar.

tests/xharness/Jenkins/TestVariationsFactory.cs

@@ -93,8 +93,6 @@ IEnumerable<TestData> GetTestData (RunTestTask test)
 			case "iPhoneSimulator":
 				switch (test.TestName) {
 				case "monotouch-test":
-					if (test.TestProject.IsDotNetProject)
-						ignore = true;
 					// The default is to run monotouch-test with the dynamic registrar (in the simulator), so that's already covered
 					yield return new TestData { Variation = "Debug (LinkSdk)", Debug = true, Profiling = false, LinkMode = "LinkSdk", Ignored = ignore };
 					yield return new TestData { Variation = "Debug (static registrar)", MTouchExtraArgs = "--registrar:static", Debug = true, Profiling = false, Undefines = "DYNAMIC_REGISTRAR", Ignored = ignore };

tools/dotnet-linker/SetupStep.cs

@@ -77,6 +77,7 @@ protected override void Process ()
 				prelink_substeps.Add (new OptimizeGeneratedCodeSubStep ());
 				prelink_substeps.Add (new MarkNSObjects ());
 				prelink_substeps.Add (new PreserveSmartEnumConversionsSubStep ());
+				prelink_substeps.Add (new CollectUnmarkedMembersSubStep ());
 
 				post_sweep_substeps.Add (new RemoveAttributesStep ());
 			}

tools/dotnet-linker/Steps/CollectUnmarkedMembers.cs

@@ -0,0 +1,37 @@
+using System.Collections.Generic;
+
+using Mono.Cecil;
+using Mono.Linker.Steps;
+
+namespace Xamarin.Linker {
+	// The static registrar may need access to information that has been linked away,
+	// in particular types and interfaces, so we need to store those somewhere
+	// so that the static registrar can access them.
+	public class CollectUnmarkedMembersSubStep : ConfigurationAwareSubStep {
+		Dictionary<TypeDefinition, List<TypeDefinition>> ProtocolImplementations => Configuration.DerivedLinkContext.ProtocolImplementations;
+
+		public override SubStepTargets Targets {
+			get {
+				return SubStepTargets.Type;
+			}
+		}
+
+		public override void ProcessType (TypeDefinition type)
+		{
+			if (!Annotations.IsMarked (type))
+				LinkContext.AddLinkedAwayType (type);
+
+			if (type.HasInterfaces) {
+				foreach (var iface in type.Interfaces) {
+					if (Annotations.IsMarked (iface))
+						continue;
+
+					// This interface might be removed, so save it
+					if (!ProtocolImplementations.TryGetValue (type, out var list))
+						ProtocolImplementations [type] = list = new List<TypeDefinition> ();
+					list.Add (iface.InterfaceType.Resolve ());
+				}
+			}
+		}
+	}
+}