不允许向前引用（forward reference）在C/C++中中很常见，即在语法上，未定义变量、类之前，不能使用。
没想到wpf中的wpf staticresource也遵循这种规则。资源字典中，进行引用时，要主要了。
详见MSDN：https://msdn.microsoft.com/en-gb/library/cc189045(v=vs.95).aspx
Important Note:
A StaticResource must not attempt to make a forward reference to a resource that is defined lexically further within the XAML file. Attempting to do so is not supported, and even if such a reference does not fail, attempting the forward reference incurs a load-time performance penalty when the internal hash tables representing a ResourceDictionary are searched. For best results, adjust the composition of your resource dictionaries such that forward references can be avoided.