Suggestions on the "Import library" fixes from dart analysis server

[yyoon]

(originally filed at Dart-Code/Dart-Code#320)

The "Import library" code fix is useful, but could be improved in a few ways.

• If the import declarations are already sorted, insert the new import in the right place. For now I have to import and then sort as a two step process.
• If the file being imported is in the current package, please consider offering the relative path import as an option.
• Also please consider offering an option to import the desired library with show Foo at the end. For instance, we do import 'dart:convert' show JSON; a lot in our project.

[srawlins]

The first two items are now implemented.

The third is tracked at #32234.