[mmp] References are optional when not linking the app, so don't show errors in that case. Fixes #40786.

mk/xamarin.mk

@@ -1,5 +1,5 @@
 ifdef ENABLE_XAMARIN
-NEEDED_MACCORE_VERSION := 9a5e6f02f305d1b580229b37c7775c4897738f4d
+NEEDED_MACCORE_VERSION := 190c67d8551def7bef9aa89c3100805ca57761ab
 NEEDED_MACCORE_BRANCH := master
 
 MACCORE_DIRECTORY := maccore

tools/mmp/Tuning.cs

@@ -123,7 +123,7 @@ static Pipeline CreatePipeline (LinkerOptions options)
 		{
 			var pipeline = new Pipeline ();
 
-			pipeline.AppendStep (new LoadReferencesStep ());
+			pipeline.AppendStep (options.LinkMode == LinkMode.None ? new LoadOptionalReferencesStep () : new LoadReferencesStep ());
 
 			if (options.I18nAssemblies != I18nAssemblies.None)
 				pipeline.AppendStep (new LoadI18nAssemblies (options.I18nAssemblies));
@@ -240,4 +240,31 @@ protected override void ProcessAssembly (AssemblyDefinition assembly)
 			base.ProcessAssembly (assembly);
 		}
 	}
+
+	class LoadOptionalReferencesStep : LoadReferencesStep
+	{
+		HashSet<AssemblyNameDefinition> _references = new HashSet<AssemblyNameDefinition> ();
+
+		protected override void ProcessAssembly (AssemblyDefinition assembly)
+		{
+			ProcessReferences (assembly);
+		}
+
+		void ProcessReferences (AssemblyDefinition assembly)
+		{
+			if (_references.Contains (assembly.Name))
+				return;
+
+			_references.Add (assembly.Name);
+
+			foreach (AssemblyNameReference reference in assembly.MainModule.AssemblyReferences) {
+				try {
+					ProcessReferences (Context.Resolve (reference));
+				} catch (AssemblyResolutionException fnfe) {
+					ErrorHelper.Warning (2013, fnfe, "Failed to resolve the reference to \"{0}\", referenced in \"{1}\". The app will not include the referenced assembly, and may fail at runtime.", fnfe.AssemblyReference.FullName, assembly.Name.FullName);
+				}
+			}
+		}
+	}
+
 }

tools/mmp/error.cs

@@ -62,6 +62,7 @@ namespace Xamarin.Bundler {
 	//					MM2010	Unknown HttpMessageHandler `{0}`. Valid values are HttpClientHandler (default), CFNetworkHandler or NSUrlSessionHandler
 	//					MM2011  Unknown TLSProvider `{0}.  Valid values are default, legacy or appletls
 	//					MM2012   Only first {0} of {1} "Referenced by" warnings shown. ** This message related to 2009 **
+	//			Warning	MM2013	Failed to resolve the reference to "{0}", referenced in "{1}". The app will not include the referenced assembly, and may fail at runtime.
 	// MM4xxx	code generation
 	// 			MM40xx	driver.m
 	//					MM4001	The main template could not be expansed to `{0}`.

tools/mtouch/error.cs

@@ -198,6 +198,7 @@ namespace Xamarin.Bundler {
 	//					MT2010	Unknown HttpMessageHandler `{0}`. Valid values are HttpClientHandler (default), CFNetworkHandler or NSUrlSessionHandler
 	//					MT2011	Unknown TlsProvider `{0}`.  Valid values are default, legacy or appletls.
 	//					MT2012  ** reserved Xamarin.Mac **
+	//					MT2013	** reserved Xamarin.Mac **
 	// MT3xxx	AOT
 	//			MT30xx	AOT (general) errors
 	//					MT3001	Could not AOT the assembly '{0}'