React to aspnet/Razor#352.

- Moved initialization of TagHelperDescriptorResolver to be Lazy so DesignTimeMode can be set.

src/Microsoft.AspNet.Mvc.Razor.Host/MvcRazorHost.cs

@@ -13,6 +13,7 @@ using Microsoft.AspNet.Razor.Chunks;
 using Microsoft.AspNet.Razor.CodeGenerators;
 using Microsoft.AspNet.Razor.Parser;
 using Microsoft.AspNet.Razor.Runtime.TagHelpers;
+using Microsoft.AspNet.Razor.TagHelpers;
 using Microsoft.Framework.Internal;
 
 namespace Microsoft.AspNet.Mvc.Razor
@@ -44,6 +45,7 @@ namespace Microsoft.AspNet.Mvc.Razor
         private readonly IChunkTreeCache _chunkTreeCache;
         private readonly RazorPathNormalizer _pathNormalizer;
         private ChunkInheritanceUtility _chunkInheritanceUtility;
+        private ITagHelperDescriptorResolver _tagHelperDescriptorResolver;
 
         internal MvcRazorHost(IChunkTreeCache chunkTreeCache, RazorPathNormalizer pathNormalizer)
             : base(new CSharpRazorCodeLanguage())
@@ -52,7 +54,6 @@ namespace Microsoft.AspNet.Mvc.Razor
             _baseType = BaseType;
             _chunkTreeCache = chunkTreeCache;
 
-            TagHelperDescriptorResolver = new TagHelperDescriptorResolver();
             DefaultBaseClass = BaseType + "<" + DefaultModel + ">";
             DefaultNamespace = "Asp";
             // Enable instrumentation by default to allow precompiled views to work with BrowserLink.
@@ -132,6 +133,27 @@ namespace Microsoft.AspNet.Mvc.Razor
         {
         }
 
+        /// <inheritdoc />
+        public override ITagHelperDescriptorResolver TagHelperDescriptorResolver
+        {
+            get
+            {
+                // The initialization of the _tagHelperDescriptorResolver needs to be lazy to allow for the setting
+                // of DesignTimeMode.
+                if (_tagHelperDescriptorResolver == null)
+                {
+                    _tagHelperDescriptorResolver = new TagHelperDescriptorResolver(DesignTimeMode);
+                }
+
+                return _tagHelperDescriptorResolver;
+            }
+            [param: NotNull]
+            set
+            {
+                _tagHelperDescriptorResolver = value;
+            }
+        }
+
         /// <summary>
         /// Gets the model type used by default when no model is specified.
         /// </summary>

src/Microsoft.AspNet.Mvc.Razor/Precompilation/RazorPreCompiler.cs

@@ -191,7 +191,7 @@ namespace Microsoft.AspNet.Mvc.Razor.Precompilation
 
         protected IMvcRazorHost GetRazorHost()
         {
-            var descriptorResolver = new TagHelperDescriptorResolver(TagHelperTypeResolver);
+            var descriptorResolver = new TagHelperDescriptorResolver(TagHelperTypeResolver, designTime: false);
             return new MvcRazorHost(new DefaultChunkTreeCache(FileProvider))
             {
                 TagHelperDescriptorResolver = descriptorResolver

test/Microsoft.AspNet.Mvc.Razor.Host.Test/MvcTagHelperAttributeValueCodeRendererTest.cs

@@ -29,7 +29,8 @@ namespace Microsoft.AspNet.Mvc.Razor
                 name: "MyAttribute",
                 propertyName: "SomeProperty",
                 typeName: propertyType,
-                isIndexer: false);
+                isIndexer: false,
+                usageDescriptor: null);
             var writer = new CSharpCodeWriter();
             var generatorContext = new ChunkGeneratorContext(
                 host: null,