Analyzer: When multiple imports provide the same symbol, one import should be marked as "unused"

[jbdeboer]

e.g. In Angular, the package:angular/angular.dart file exports package:di/di.dart.

In the following file, the Module symbol is coming from di.dart, but also exported through angular.

import package:angular/angular.dart
import package:di/di.dart

class MyModule extends Module { ... }

Currently, the analyzer does not give any hints about unused imports.

However, I would expect angular.dart to be flagged as "unused". angular.dart is not used since Module is also available through di.dart.

[srawlins]

Even a subset of this, examining just shown names would be useful. I found some code with:

import 'package:a/a.dart';
import 'package:a/src/foo.dart' show foo;

because at one point, a.dart did not export foo. But now it does, so the second import is unnecessary. Not sure if one is easier to implement or faster to run than the other...

[srawlins]

I'll close this in favor of the issue I've been referencing when landing changes. #44569