C# - Unit Testing Custom BuildProvider?

How can BuildProvider's GenerateCode method be tested. I created a BuildProvider and now I want to unit test it. Is it possible? Initial question was not clear enough. I don't want to stub AssemblyBuilder, I want to mimic asp.net request. I want to make sure that VirtualPathProvider and BuildProvider work fine together.

One more clarification, I am not really interested in generated code from build provider, it is used only to kick-start compilation in medium-trust environment, I already have a code, I need build provider just so I could compile since CodeDomProvider is not accessible directly in medium trust.

public override void GenerateCode(AssemblyBuilder assemblyBuilder)
{
string outputName = VirtualPath.Substring(VirtualPath.LastIndexOf('/') + 1);
outputName = outputName.Substring(0, outputName.LastIndexOf('.'));
_compilationContext = (CompilationContext)HttpContext.Current.Items[outputName];
var tw = assemblyBuilder.CreateCodeFile(this);
tw.Write(_compilationContext.Content);
tw.Flush();
foreach (var assembly in CompilerServices.GetLoadedAssemblies().Where(a => !a.IsDynamic))
assemblyBuilder.AddAssemblyReference(assembly);
}