[build] post-build fix of the assembly

- fixes #2835

ShameHat.Tasks/FixXFCoreAssembly.cs

@@ -0,0 +1,99 @@
+using System;
+using Microsoft.Build.Utilities;
+using Mono.Cecil;
+using Mono.Cecil.Rocks;
+
+using System.Linq;
+using Mono.Cecil.Cil;
+using Microsoft.Build.Framework;
+using System.IO;
+
+namespace ShameHat.Tasks
+{
+	public class FixXFCoreAssembly : Task
+	{
+		[Required]
+		public string Assembly { get; set; }
+		public string ReferencePath { get; set; }
+
+		public override bool Execute()
+		{
+			Log.LogMessage("Generating backcompat code for #2835");
+
+			var resolver =  new AssemblyResolver();
+
+			if (!string.IsNullOrEmpty(ReferencePath)) {
+				var paths = ReferencePath.Replace("//", "/").Split(';');
+				foreach (var p in paths) {
+					var searchpath = Path.GetDirectoryName(p);
+					resolver.AddSearchDirectory(searchpath);
+				}
+			}
+
+			using (var assemblyDefinition = AssemblyDefinition.ReadAssembly(Assembly, new ReaderParameters { AssemblyResolver = resolver, ReadWrite = true, ReadSymbols = true })) {
+				var resourceLoader = assemblyDefinition.MainModule.GetType("Xamarin.Forms.Internals.ResourceLoader");
+
+				var methodDef = new MethodDefinition("get_ResourceProvider",
+													 MethodAttributes.Static | MethodAttributes.Public | MethodAttributes.SpecialName,
+													 assemblyDefinition.MainModule.ImportReference(("mscorlib", "System", "Func`2"),
+				                                                                                   classArguments: new[] { ("mscorlib", "System", "String"), ("mscorlib", "System", "String") }));
+				var body = new MethodBody(methodDef);
+				var il = body.GetILProcessor();
+				il.Emit(OpCodes.Ldnull);
+				il.Emit(OpCodes.Ret);
+				methodDef.Body = body;
+				resourceLoader.Methods.Add(methodDef);
+
+				assemblyDefinition.Write(new WriterParameters {
+					WriteSymbols = true,
+				});
+			}
+			return true;
+		}
+	}
+
+	class AssemblyResolver : DefaultAssemblyResolver
+	{
+		public void AddAssembly(string p)
+		{
+			RegisterAssembly(AssemblyDefinition.ReadAssembly(p, new ReaderParameters {
+				AssemblyResolver = this
+			}));
+		}
+	}
+
+	public static class ModuleDefinitionExtensions
+	{
+		public static TypeReference ImportReference(this ModuleDefinition module, (string assemblyName, string clrNamespace, string typeName) type)
+		{
+			return module.ImportReference(module.GetTypeDefinition(type));
+		}
+
+		public static TypeReference ImportReference(this ModuleDefinition module, (string assemblyName, string clrNamespace, string typeName) type, (string assemblyName, string clrNamespace, string typeName)[] classArguments)
+		{
+			return module.ImportReference(module.ImportReference(type).MakeGenericInstanceType(classArguments.Select(gp => module.ImportReference(module.GetTypeDefinition((gp.assemblyName, gp.clrNamespace, gp.typeName)))).ToArray()));
+		}
+
+		public static TypeDefinition GetTypeDefinition(this ModuleDefinition module, (string assemblyName, string clrNamespace, string typeName) type)
+		{
+			var asm = module.Assembly.Name.Name == type.assemblyName
+							? module.Assembly
+							: module.AssemblyResolver.Resolve(AssemblyNameReference.Parse(type.assemblyName));
+			var typeDef = asm.MainModule.GetType($"{type.clrNamespace}.{type.typeName}");
+			if (typeDef != null) {
+				return typeDef;
+			}
+			var exportedType = asm.MainModule.ExportedTypes.FirstOrDefault(
+				arg => arg.IsForwarder && arg.Namespace == type.clrNamespace && arg.Name == type.typeName);
+			if (exportedType != null) {
+				typeDef = exportedType.Resolve();
+				return typeDef;
+			}
+
+			//I hate you, netstandard
+			if (type.assemblyName == "mscorlib" && type.clrNamespace == "System.Reflection")
+				return module.GetTypeDefinition(("System.Reflection", type.clrNamespace, type.typeName));
+			return null;
+		}
+	}
+}

ShameHat.Tasks/ShameHat.Tasks.csproj

@@ -0,0 +1,15 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>netstandard2.0</TargetFramework>
+		<CopyLocalLockFileAssemblies>true</CopyLocalLockFileAssemblies>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.Build" Version="15.7.179" />
+    <PackageReference Include="Microsoft.Build.Framework" Version="15.7.179" />
+    <PackageReference Include="Microsoft.Build.Utilities.Core" Version="15.7.179" />
+    <PackageReference Include="Microsoft.Build.Tasks.Core" Version="15.7.179" />
+    <PackageReference Include="Mono.Cecil" Version="0.10.0-beta7" />
+  </ItemGroup>
+</Project>