Add System.Xml.dll to unit test references

ICSharpCode.Decompiler.Tests/Helpers/Tester.cs

@@ -154,7 +154,8 @@ namespace ICSharpCode.Decompiler.Tests.Helpers
 			{
 					MetadataReference.CreateFromFile(Path.Combine(refAsmPath, "mscorlib.dll")),
 					MetadataReference.CreateFromFile(Path.Combine(refAsmPath, "System.dll")),
-					MetadataReference.CreateFromFile(Path.Combine(refAsmPath, "System.Core.dll"))
+					MetadataReference.CreateFromFile(Path.Combine(refAsmPath, "System.Core.dll")),
+					MetadataReference.CreateFromFile(Path.Combine(refAsmPath, "System.Xml.dll"))
 			};
 		});
 		
@@ -222,7 +223,9 @@ namespace ICSharpCode.Decompiler.Tests.Helpers
 					options.OutputAssembly = outputFileName;
 				}
 
+				options.ReferencedAssemblies.Add("System.dll");
 				options.ReferencedAssemblies.Add("System.Core.dll");
+				options.ReferencedAssemblies.Add("System.Xml.dll");
 				CompilerResults results = provider.CompileAssemblyFromFile(options, sourceFileNames.ToArray());
 				if (results.Errors.Cast<CompilerError>().Any(e => !e.IsWarning)) {
 					StringBuilder b = new StringBuilder("Compiler error:");