Extension types should be able to omit their bodys.

Currently, extension types can't omit their body.
```An extension type declaration must have a body, even if it is empty.```

As a *smart* alternative of `typedef`, onliner terminated by semicolon seems important.

Start by locating the Dart language specification rule quoted in the issue and read the surrounding extension type declaration rules. Compare the proposed semicolon-terminated form with typedef syntax; done means an extension type can omit its body while retaining valid declaration and parsing rules.

Written by the indexing model from the issue text.