[Fixes #472] Added temporary parse error for helper directive

Author: ajaybhargavb

src/Microsoft.AspNet.Razor/Parser/CSharpCodeParser.cs

@@ -224,6 +224,14 @@ private void AfterTransition()
                             }
                             else
                             {
+                                if (CurrentSymbol.Content.Equals(SyntaxConstants.CSharp.HelperKeyword))
+                                {
+                                    Context.OnError(
+                                        CurrentLocation,
+                                        RazorResources.FormatParseError_HelperDirectiveNotAvailable(SyntaxConstants.CSharp.HelperKeyword),
+                                        CurrentSymbol.Content.Length);
+                                }
+
                                 Context.CurrentBlock.Type = BlockType.Expression;
                                 Context.CurrentBlock.ChunkGenerator = new ExpressionChunkGenerator();
                                 ImplicitExpression();

src/Microsoft.AspNet.Razor/Parser/SyntaxConstants.cs

@@ -27,6 +27,9 @@ public static class CSharp
             public static readonly string ElseIfKeyword = "else if";
             public static readonly string NamespaceKeyword = "namespace";
             public static readonly string ClassKeyword = "class";
+
+            // Not supported. Only used for error cases.
+            public static readonly string HelperKeyword = "helper";
         }
     }
 }

src/Microsoft.AspNet.Razor/Properties/RazorResources.Designer.cs

@@ -425,4 +425,7 @@ Instead, wrap the contents of the block in "{{}}":
   <data name="TagHelperParseTreeRewriter_InvalidNestedTag" xml:space="preserve">
     <value>The &lt;{0}&gt; tag is not allowed by parent &lt;{1}&gt; tag helper. Only child tag helper(s) targeting tag name(s) '{2}' are allowed.</value>
   </data>
+  <data name="ParseError_HelperDirectiveNotAvailable" xml:space="preserve">
+    <value>The {0} directive is not supported.</value>
+  </data>
 </root>

src/Microsoft.AspNet.Razor/RazorResources.resx

@@ -27,6 +27,21 @@ public void ParseBlockHandlesQuotesAfterTransition()
                                length: 1));
         }
 
+        [Fact]
+        public void ParseBlockWithHelperDirectiveProducesError()
+        {
+            ParseBlockTest("@helper fooHelper { }",
+                new ExpressionBlock(
+                    Factory.CodeTransition(),
+                    Factory.Code("helper")
+                        .AsImplicitExpression(KeywordSet)
+                        .Accepts(AcceptedCharacters.NonWhiteSpace)),
+                new RazorError(
+                    RazorResources.FormatParseError_HelperDirectiveNotAvailable(SyntaxConstants.CSharp.HelperKeyword),
+                    new SourceLocation(1, 0, 1),
+                    length: 6));
+        }
+
         [Fact]
         public void ParseBlockCapturesWhitespaceToEndOfLineInInvalidUsingStatementAndTreatsAsFileCode()
         {