Remove DesignTimeHost dependency.

- This resulted in us decoupling ourselves from the `IPlugin` interface. It's going away.
- Redesigned the `TagHelperDescriptor` resolver to be a glorified factory with a protocol. It used to serialize/deserialize messages as input because it was an `IPlugin`.

#25

src/Microsoft.AspNet.Tooling.Razor/AssemblyTagHelperDescriptorResolver.cs

@@ -0,0 +1,54 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Microsoft.AspNet.Razor;
+using Microsoft.AspNet.Razor.Compilation.TagHelpers;
+using Microsoft.AspNet.Razor.Runtime.TagHelpers;
+using Microsoft.Extensions.PlatformAbstractions;
+
+namespace Microsoft.AspNet.Tooling.Razor
+{
+    public class AssemblyTagHelperDescriptorResolver
+    {
+        public int Protocol { get; set; } = 1;
+
+        public IEnumerable<TagHelperDescriptor> Resolve(
+            string assemblyName,
+            IAssemblyLoadContext assemblyLoadContext,
+            ErrorSink errorSink)
+        {
+            if (assemblyName == null)
+            {
+                throw new ArgumentNullException(nameof(assemblyName));
+            }
+
+            if (assemblyLoadContext == null)
+            {
+                throw new ArgumentNullException(nameof(assemblyLoadContext));
+            }
+
+            if (Protocol == 1)
+            {
+                var tagHelperTypeResolver = new AssemblyLoadContextTagHelperTypeResolver(assemblyLoadContext);
+                var tagHelperTypes = tagHelperTypeResolver.Resolve(assemblyName, SourceLocation.Zero, errorSink);
+                var tagHelperDescriptors = tagHelperTypes.SelectMany(
+                    type => TagHelperDescriptorFactory.CreateDescriptors(
+                        assemblyName,
+                        type,
+                        designTime: true,
+                        errorSink: errorSink));
+
+                return tagHelperDescriptors;
+            }
+            else
+            {
+                // Unknown protocol
+                throw new InvalidOperationException(
+                    Resources.FormatInvalidProtocolValue(typeof(TagHelperDescriptor).FullName, Protocol));
+            }
+        }
+    }
+}

src/Microsoft.AspNet.Tooling.Razor/ResolveProtocolCommand.cs → src/Microsoft.AspNet.Tooling.Razor/Internal/ResolveProtocolCommand.cs

@@ -4,11 +4,11 @@
 using System;
 using Microsoft.Dnx.Runtime.Common.CommandLine;
 
-namespace Microsoft.AspNet.Tooling.Razor
+namespace Microsoft.AspNet.Tooling.Razor.Internal
 {
-    internal static class ResolveProtocolCommand
+    public static class ResolveProtocolCommand
     {
-        public static void Register(CommandLineApplication app)
+        internal static void Register(CommandLineApplication app)
         {
             app.Command("resolve-protocol", config =>
             {
@@ -20,7 +20,7 @@ public static void Register(CommandLineApplication app)
 
                 config.OnExecute(() =>
                 {
-                    var pluginProtocol = new RazorPlugin(messageBroker: null).Protocol;
+                    var pluginProtocol = new AssemblyTagHelperDescriptorResolver().Protocol;
                     var clientProtocolString = clientProtocolArgument.Value;
                     var clientProtocol = int.Parse(clientProtocolString);
                     var resolvedProtocol = ResolveProtocol(clientProtocol, pluginProtocol);
@@ -35,7 +35,7 @@ public static void Register(CommandLineApplication app)
         /// <summary>
         /// Internal for testing.
         /// </summary>
-        internal static int ResolveProtocol(int clientProtocol, int pluginProtocol)
+        public static int ResolveProtocol(int clientProtocol, int pluginProtocol)
         {
             // Protocols start at 1 and increase.
             clientProtocol = Math.Max(1, clientProtocol);

src/Microsoft.AspNet.Tooling.Razor/Internal/ResolveTagHelpersCommand.cs

@@ -0,0 +1,71 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Microsoft.AspNet.Razor;
+using Microsoft.AspNet.Razor.Compilation.TagHelpers;
+using Microsoft.Dnx.Runtime.Common.CommandLine;
+using Microsoft.Extensions.PlatformAbstractions;
+using Newtonsoft.Json;
+
+namespace Microsoft.AspNet.Tooling.Razor.Internal
+{
+    public static class ResolveTagHelpersCommand
+    {
+        internal static void Register(CommandLineApplication app, IAssemblyLoadContext assemblyLoadContext)
+        {
+            app.Command("resolve-taghelpers", config =>
+            {
+                config.Description = "Resolves TagHelperDescriptors in the specified assembly(s).";
+                config.HelpOption("-?|-h|--help");
+                var protocolOption = config.Option(
+                    "-p|--protocol",
+                    "Protocol to resolve TagHelperDescriptors with.",
+                    CommandOptionType.SingleValue);
+                var assemblyNames = config.Argument(
+                    "[name]",
+                    "Assembly name to resolve TagHelperDescriptors in.",
+                    multipleValues: true);
+
+                config.OnExecute(() =>
+                {
+                    var protocol = int.Parse(protocolOption.Value());
+
+                    var descriptorResolver = new AssemblyTagHelperDescriptorResolver
+                    {
+                        Protocol = protocol
+                    };
+
+                    var errorSink = new ErrorSink();
+                    var resolvedDescriptors = new List<TagHelperDescriptor>();
+                    for (var i = 0; i < assemblyNames.Values.Count; i++)
+                    {
+                        var assemblyName = assemblyNames.Values[i];
+                        var descriptors = descriptorResolver.Resolve(assemblyName, assemblyLoadContext, errorSink);
+                        resolvedDescriptors.AddRange(descriptors);
+                    }
+
+                    var resolvedResult = new ResolvedTagHelperDescriptorsResult
+                    {
+                        Descriptors = resolvedDescriptors,
+                        Errors = errorSink.Errors
+                    };
+                    var serializedResult = JsonConvert.SerializeObject(resolvedResult, Formatting.Indented);
+
+                    Console.WriteLine(serializedResult);
+
+                    return errorSink.Errors.Any() ? 1 : 0;
+                });
+            });
+        }
+
+        private class ResolvedTagHelperDescriptorsResult
+        {
+            public IEnumerable<TagHelperDescriptor> Descriptors { get; set; }
+
+            public IEnumerable<RazorError> Errors { get; set; }
+        }
+    }
+}

src/Microsoft.AspNet.Tooling.Razor/Program.cs

@@ -3,8 +3,9 @@
 
 using System;
 using System.Reflection;
-using Microsoft.Extensions.PlatformAbstractions;
+using Microsoft.AspNet.Tooling.Razor.Internal;
 using Microsoft.Dnx.Runtime.Common.CommandLine;
+using Microsoft.Extensions.PlatformAbstractions;
 
 namespace Microsoft.AspNet.Tooling.Razor
 {

src/Microsoft.AspNet.Tooling.Razor/Properties/AssemblyInfo.cs

@@ -5,6 +5,5 @@
 using System.Resources;
 using System.Runtime.CompilerServices;
 
-[assembly: InternalsVisibleTo("Microsoft.AspNet.Tooling.Razor.Test, PublicKey=0024000004800000940000000602000000240000525341310004000001000100f33a29044fa9d740c9b3213a93e57c84b472c84e0b8a0e1ae48e67a9f8f6de9d5f7f3d52ac23e48ac51801f1dc950abe901da34d2a9e3baadb141a17c77ef3c565dd5ee5054b91cf63bb3c6ab83f72ab3aafe93d0fc3c2348b764fafb0b1c0733de51459aeab46580384bf9d74c4e28164b7cde247f891ba07891c9d872ad2bb")]
 [assembly: AssemblyMetadata("Serviceable", "True")]
 [assembly: NeutralResourcesLanguage("en-us")]