Bad autocomplete behavior

There are a few issues in this .gif.

1. As I start to type 'my' I would expect autocomplete to suggest the local variables above as autocompletions.
2. Once I type `myMap` and hit `.`, autocomplete results come up for `myMap`. Once I backspace, these autocomplete suggestions should go away.
3. As I start typing something new, the stale autocomplete results from `myMap` are still showing up when they shouldn't be.

@elliette @polina-c

首先复现 issue 中描述的三个 autocomplete 案例，并将它们与附带的 GIF 进行比较。跟踪负责局部变量建议和结果失效的 autocomplete 实现；当能够建议局部变量，并且在退格删除或开始新的输入后过时结果消失时，即表示完成。

由索引模型根据 Issue 内容生成。