Report unused public elements in an entrypoint library

[srawlins]

Describe the rule you'd like to see implemented

If a library has an entrypoint, I'd like to report any publicly accessible elements which are not used within the library as unused.

An entrypoint is a top-level main function, or a function annotated with @pragma(vm:entry-point). These files are typically:

• a VM script in bin/,
• a web app in web/,
• a VM tool in tool/,
• a test like test/**/*_test.dart,

I think there will be very very few false positives. And perhaps any false positive indicates a code smell in library arrangement (the analyzer codebase I believe has some test files which reference another test file, each of which has a main).

Any element which is not publicly visible should already be reported with analyzer's unused_local_variable, unused_element, or unused_field diagnostics. This lint would cover publicly visible elements:

• Any public class instance members (fields, methods, constructors), even those on private classes and mixins.
• Any public top-level functions, variables, classes, enums, mixins, extensions, and typedefs.

Examples

// foo_test.dart

void main() {
  test(...);
}

// Unused class
class MockStream extends Mock implements Stream {
  // Unused field
  int y = 7;

  // Unused method
  void m() {}
}

// Unused top-level function
void helper() {}

// Unused top-level variable
const x = 1;

Additional context

This is an alternative route to #57580 and #57824.

[goderbauer]

As a data point: Recently did some clean-up in the framework with the help of this tool: https://pub.dev/packages/dart_code_metrics. We had accumulated quite a bit of dead code in tests: flutter/flutter#104550.

This proposal (or #57824 or #57580) probably could have prevented that build-up of dead code.