Upgrade to v3.5: Compiler executable file csc.exe cannot be found.

Server Error in '/' Application.
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Compiler executable file csc.exe cannot be found. 
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 
 
Exception Details: System.InvalidOperationException: Compiler executable file csc.exe cannot be found.
 
Source Error: 
 
An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.  
 
Stack Trace: 
 
 
[InvalidOperationException: Compiler executable file csc.exe cannot be found.]
   System.CodeDom.Compiler.RedistVersionInfo.GetCompilerPath(IDictionary`2 provOptions, String compilerExecutable) +2191394
   Microsoft.CSharp.CSharpCodeGenerator.FromFileBatch(CompilerParameters options, String[] fileNames) +562
   Microsoft.CSharp.CSharpCodeGenerator.System.CodeDom.Compiler.ICodeCompiler.CompileAssemblyFromFileBatch(CompilerParameters options, String[] fileNames) +185
   System.CodeDom.Compiler.CodeDomProvider.CompileAssemblyFromFile(CompilerParameters options, String[] fileNames) +23
   System.Web.Compilation.AssemblyBuilder.Compile() +891
   System.Web.Compilation.BuildProvidersCompiler.PerformBuild() +213
   System.Web.Compilation.ApplicationBuildProvider.GetGlobalAsaxBuildResult(Boolean isPrecompiledApp) +269
   System.Web.Compilation.BuildManager.CompileGlobalAsax() +47
   System.Web.Compilation.BuildManager.EnsureTopLevelFilesCompiled() +510
 
[HttpException (0x80004005): Compiler executable file csc.exe cannot be found.]
   System.Web.Compilation.BuildManager.ReportTopLevelCompilationException() +58
   System.Web.Compilation.BuildManager.EnsureTopLevelFilesCompiled() +610
   System.Web.Hosting.HostingEnvironment.Initialize(ApplicationManager appManager, IApplicationHost appHost, IConfigMapPathFactory configMapPathFactory, HostingEnvironmentParameters hostingParameters) +803
 
[HttpException (0x80004005): Compiler executable file csc.exe cannot be found.]
   System.Web.HttpRuntime.FirstRequestInit(HttpContext context) +940
   System.Web.HttpRuntime.EnsureFirstRequestInit(HttpContext context) +70
   System.Web.HttpRuntime.ProcessRequestInternal(HttpWorkerRequest wr) +275