Addressed code review comments.

Author: NTaylorMullen

src/Microsoft.AspNet.Razor.Runtime/Properties/Resources.Designer.cs

@@ -117,12 +117,12 @@
   <resheader name="writer">
     <value>System.Resources.ResXResourceWriter, System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
   </resheader>
-  <data name="TagHelperDescriptorResolver_InvalidTagHelperLookupText" xml:space="preserve">
-    <value>Invalid TagHelper lookup text '{0}'. The correct lookup text formats are:
+  <data name="TagHelperDescriptorResolver_InvalidAddTagHelperLookupText" xml:space="preserve">
+    <value>Invalid `@addtaghelper` lookup text '{0}'. The correct lookup text formats are:
 "assemblyName"
 "typeName, assemblyName"</value>
   </data>
-  <data name="TagHelperTypeResolver_AssemblyNameCannotBeEmptyOrNull" xml:space="preserve">
-    <value>Assembly name lookup text cannot be null or empty.</value>
+  <data name="TagHelperTypeResolver_AddTagHelperAssemblyNameCannotBeEmptyOrNull" xml:space="preserve">
+    <value>`@addtaghelper` assembly name cannot be null or empty.</value>
   </data>
 </root>

src/Microsoft.AspNet.Razor.Runtime/Resources.resx

@@ -10,12 +10,11 @@
 namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
 {
     /// <summary>
-    /// Defines a class that can be used to create <see cref="TagHelperDescriptor"/>s from <see cref="Type"/>s.
+    /// Factory for <see cref="TagHelperDescriptor"/>s from <see cref="Type"/>s.
     /// </summary>
     public static class TagHelperDescriptorFactory
     {
         private const string TagHelperNameEnding = "TagHelper";
-        private const ContentBehavior DefaultContentBehavior = ContentBehavior.None;
 
         // TODO: Investigate if we should cache TagHelperDescriptors for types:
         // https://github.com/aspnet/Razor/issues/165
@@ -70,7 +69,7 @@ private static TagHelperAttributeDescriptor ToTagHelperAttributeDescriptor(Prope
         // TODO: Make the content behavior pull from a ContentBehaviorAttribute: https://github.com/aspnet/Razor/issues/122
         private static ContentBehavior GetContentBehavior(Type type)
         {
-            return DefaultContentBehavior;
+            return ContentBehavior.None;
         }
 
         private static bool IsValidTagHelperProperty(PropertyInfo property)

src/Microsoft.AspNet.Razor.Runtime/TagHelpers/TagHelperDescriptorFactory.cs

@@ -35,7 +35,7 @@ public IEnumerable<TagHelperDescriptor> Resolve(string lookupText)
             if (string.IsNullOrEmpty(lookupText))
             {
                 throw new InvalidOperationException(
-                    Resources.FormatTagHelperDescriptorResolver_InvalidTagHelperLookupText(lookupText));
+                    Resources.FormatTagHelperDescriptorResolver_InvalidAddTagHelperLookupText(lookupText));
             }
 
             var tagHelperTypes = ResolveTagHelperTypes(lookupText);
@@ -53,10 +53,10 @@ private IEnumerable<Type> ResolveTagHelperTypes(string lookupText)
             // Ensure that we have valid data to work with. Valid formats are:
             // "assemblyName"
             // "typeName, assemblyName"
-            if (data.Length > 2)
+            if (data.Length != 1 && data.Length != 2)
             {
                 throw new InvalidOperationException(
-                    Resources.FormatTagHelperDescriptorResolver_InvalidTagHelperLookupText(lookupText));
+                    Resources.FormatTagHelperDescriptorResolver_InvalidAddTagHelperLookupText(lookupText));
             }
 
             // Grab the assembly name from the lookup text data.

src/Microsoft.AspNet.Razor.Runtime/TagHelpers/TagHelperDescriptorResolver.cs

@@ -32,7 +32,8 @@ public IEnumerable<Type> Resolve(string assemblyName)
         {
             if (string.IsNullOrEmpty(assemblyName))
             {
-                throw new InvalidOperationException(Resources.TagHelperTypeResolver_AssemblyNameCannotBeEmptyOrNull);
+                throw new InvalidOperationException(
+                    Resources.TagHelperTypeResolver_AddTagHelperAssemblyNameCannotBeEmptyOrNull);
             }
 
             var assemblyRef = new AssemblyName(assemblyName);

src/Microsoft.AspNet.Razor.Runtime/TagHelpers/TagHelperTypeResolver.cs

@@ -157,7 +157,8 @@ public void DescriptorResolver_ResolveThrowsIfNullOrEmptyLookupText(string looku
                 new TagHelperDescriptorResolver(
                     new TestTagHelperTypeResolver(InvalidTestableTagHelpers));
 
-            var expectedMessage = Resources.FormatTagHelperDescriptorResolver_InvalidTagHelperLookupText(lookupText);
+            var expectedMessage = 
+                Resources.FormatTagHelperDescriptorResolver_InvalidAddTagHelperLookupText(lookupText);
 
             // Act & Assert
             var ex = Assert.Throws<InvalidOperationException>(() =>

test/Microsoft.AspNet.Razor.Runtime.Test/TagHelpers/TagHelperDescriptorResolverTest.cs

@@ -67,7 +67,7 @@ public void TypeResolver_ResolveThrowsIfEmptyOrNullLookupText(string assemblyNam
         {
             // Arrange
             var tagHelperTypeResolver = new TestTagHelperTypeResolver(InvalidTestableTagHelpers);
-            var expectedMessage = "Assembly name lookup text cannot be null or empty.";
+            var expectedMessage = "`@addtaghelper` assembly name cannot be null or empty.";
 
             // Act & Assert
             var ex = Assert.Throws<InvalidOperationException>(() =>