Remove unused importDRT-W1696

The analyzer produces this diagnostic when an import isn't needed because none of the names that are imported are referenced within the importing library.

Example

The following code produces this diagnostic because nothing defined in dart:async is referenced in the library:

import 'dart:async';

void main() {}

Common fixes

If the import isn't needed, then remove it.

If some of the imported names are intended to be used, then add the missing code.