Razor intellisense error: Feature ‘extension method’ cannot be used because it is not part of the ISO-2 C# language specification

The key of solving this issue is adding the following tag to your app.config or web.config

<system.web>
    <compilation debug="false" targetFramework="4.0" />
</system.web>

Note: In some cases, unloading and reloading the project is required.