Added support for @Helper and @section.

Author: NTaylorMullen

src/Microsoft.AspNet.Razor/Generator/Compiler/CodeBuilder/CSharp/CSharpTagHelperCodeRenderer.cs

@@ -310,11 +310,8 @@ private void RenderTagHelperBody(IList<Chunk> children, ContentBehavior contentB
             // If content behavior is modify we need to use a different writer to buffer the body.
             if (contentBehavior == ContentBehavior.Modify)
             {
-                BuildBufferedWritingScope(() =>
-                {
-                    // Render all of the tag helper children
-                    _bodyVisitor.Accept(children);
-                });
+                // Render all of the tag helper children in a buffered writing scope
+                BuildBufferedWritingScope(children);
             }
             else
             {
@@ -401,21 +398,27 @@ private void RenderQuotedAttributeValue(string value, TagHelperAttributeDescript
 
         private void BuildBufferedAttributeValue(Chunk htmlAttributeChunk)
         {
-            BuildBufferedWritingScope(renderCode: () =>
-            {
-                // Render the HTML's attribute chunk(s).
-                _bodyVisitor.Accept(htmlAttributeChunk);
-            });
+            // Render a buffered writing scope for the html attribute value.
+            BuildBufferedWritingScope(new[] { htmlAttributeChunk });
         }
 
-        private void BuildBufferedWritingScope(Action renderCode)
+        private void BuildBufferedWritingScope(IList<Chunk> chunks)
         {
+            // We're building a writing scope around the provided chunks which captures everything written  from the
+            // page. Therefore, we do not want to write to any other buffer since we're using the pages buffer to 
+            // ensure we capture all content that's written, directly or indirectly.
+            var oldWriter = _context.TargetWriterName;
+            _context.TargetWriterName = null;
+
             _writer.WriteMethodInvocation(_tagHelperContext.StartWritingScopeMethodName);
 
-            renderCode();
+            _bodyVisitor.Accept(chunks);
 
             _writer.WriteStartAssignment(StringValueBufferVariableName)
                    .WriteMethodInvocation(_tagHelperContext.EndWritingScopeMethodName);
+
+            // Reset the writer/buffer back to what it was, no longer in a writing scope.
+            _context.TargetWriterName = oldWriter;
         }
 
         private void RenderAttributeValue(TagHelperAttributeDescriptor attributeDescriptor,