Add "sealed" key word to mark some public/protected methods not be overriden

还没有人认领这个 Issue。

From https://github.com/Microsoft/TypeScript/issues/1524, I see that the default method are all "virtual", but sometimes I don't wanna some public method to be overidden. Would you invent a mechasim to force the users that they cannot override a public method (with the key word, something like "sealed")?

Thanks!

首先阅读链接的 TypeScript issue #1524 以及此 issue 中的讨论，以了解 sealed method 的提议含义。定义语言和类型检查行为，包括应如何拒绝重写，然后在实现之前确定相关的编译器测试。行为完成规范定义并由测试覆盖后，即视为完成。

由索引模型根据 Issue 内容生成。