fix #537: ILSpy can't find any Overridden By methods

Overriding methods were not shown because the base method was marked
'internal' and so would normally only be visible within its own
assembly.
However the assembly had a InternalsVisibleToAttribute specified which
allowed access from the assembly containing the derived types....

ICSharpCode.Decompiler/Ast/TypesHierarchyHelpers.cs

@@ -185,24 +185,28 @@ namespace ICSharpCode.Decompiler.Ast
 			if (derivedType == null)
 				throw new ArgumentNullException("derivedType");
 
-			var visibility = IsVisibleFromDerived(baseMember);
+			MethodAttributes attrs = GetAccessAttributes(baseMember) & MethodAttributes.MemberAccessMask;
-			if (visibility.HasValue)
+			if (attrs == MethodAttributes.Private)
-				return visibility.Value;
+				return false;
 
-			if (baseMember.DeclaringType.Module == derivedType.Module)
+			if (attrs == MethodAttributes.Assembly || attrs == MethodAttributes.FamANDAssem) {
+				var derivedTypeAsm = derivedType.Module.Assembly;
+				var asm = baseMember.DeclaringType.Module.Assembly;
+				if (asm.HasCustomAttributes) {
+					var attributes = asm.CustomAttributes
+						.Where(attr => attr.AttributeType.FullName == "System.Runtime.CompilerServices.InternalsVisibleToAttribute");
+					foreach (var attribute in attributes) {
+						string assemblyName = attribute.ConstructorArguments[0].Value as string;
+						assemblyName = assemblyName.Split(',')[0]; // strip off any public key info
+						if (assemblyName == derivedTypeAsm.Name.Name)
 							return true;
-			// TODO: Check also InternalsVisibleToAttribute.
+					}
+				}
 				return false;
 			}
 
-		private static bool? IsVisibleFromDerived(IMemberDefinition member)
-		{
-			MethodAttributes attrs = GetAccessAttributes(member) & MethodAttributes.MemberAccessMask;
-			if (attrs == MethodAttributes.Private)
-				return false;
-			if (attrs == MethodAttributes.Assembly || attrs == MethodAttributes.FamANDAssem)
-				return null;
 			return true;
+
 		}
 
 		private static MethodAttributes GetAccessAttributes(IMemberDefinition member)