After further investigation I found that this is a known issue.
source: https://support.microsoft.com/en-us/kb/3025133
ASP.NET and Web Development
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When you create a Web Forms 4.5 WAP and open a Web Form page, you receive the following errors in the Errors List window:
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The project will run without any issues. Error CS0234 The type or namespace name ‘global_asax’ does not exist in the namespace ‘ASP’ (are you missing an assembly reference?)
Error CS0234 The type or namespace name ‘Linq’ does not exist in the namespace ‘System’ (are you missing an assembly reference?)
Assume that you use new language features for C# and VB in Visual Studio 2015 RC. You receive a runtime error when you use C# or VB in a Web Form page or in Razor Views.
To work around this issue, install Microsoft.CodeDom.Providers.DotNetCompilerPlatform NuGet package. This package will substitute the Roslyn-based provider for the in-box CodeDom providers for ASP.NET.