Add autocompletion for first created class in file

[ph55]

Feature request/improvement.

Please add autocompletion for first created class in file.
The default class name could suggest class name as PascalCase of file name without separators (underscores, hyphens, dots, etc).

Currently nothing happens:

It might suggest ConfigService.

You can see such behaviour in PHPStorm with php classes and WebStorm with JS/TS classes.

This issue was moved by pq to dart-lang/sdk#42360.