SInce this is a subset of `always_specify_types` I think we can safely remove it.
In practice, if you violate you get doubly nagged:
```
[lint] Type annotate public APIs. (packages/flutter_tools/lib/src/dart/runner.dart, line 5, col 1)
[lint] Declare method return types. (packages/flutter_tools/lib/src/dart/runner.dart, line 5, col 1)
```
One warning is probably enough to get the message across? ;)
It's safe to remove the unneeded `void`s from setters since the blocking issues in the
`always_declare_return_types` lint have been fixed (https://github.com/dart-lang/linter/). We can also safely flip the bit on `avoid_return_types_on_setters`.
* Enable the lint require documentation.
* Track how many public Flutter members lack docs
* Rename the .analysis_options file
This makes Atom happier in the flutter_tools package.
