[dart2js] Unused fields are not tree-shaken off, sensitive to type annotations

[yjbanov]

INPUT:

main() {
  print(new Foo().hello);
}

class Bar {
  String txt = " World";
}

class Foo {
  String _hello;
  Bar _world; // never initialized
  String get hello {
    if (_hello == null) {
      _hello = "Hello";
    }
    return _hello;
  }
  String get world {
    if (_world == null) {
      _world = new Bar();
    }
    return _world.txt;
  }
}

OUTPUT (reformatted):

main(){
  print(new Foo().hello);
}

class Bar{} // Expected Bar to be gone.
             // However, it will be gone if I change type of _world to "var".
             // Nice to see txt tree-shaken off.

class Foo{
  String _hello; // Used, so it stays.
  Bar _world; // Not used, expected to be gone
  String get hello{
    if(_hello==null) {
      _hello="Hello";
    }
    return _hello;
  }
}

[anders-sandholm]

Added Area-Dart2Dart, Triaged labels.

[yjbanov]

This issue was misclassified. Reopening and moving to dart2js.

[greglittlefield-wf]

I just ran into the issue, only I was seeing it where unused fields from the superclass overridden by getters/setters in a subclass are not completely tree-shaken, and are still present in the JS constructor initializer list.

For example,

class A {
  int a;
}
class B extends A {
  int get a => ...;
  set a(int value) => ...;
}

For more context on my use-case, see the "Leaving behind fields" section here: dart-lang/build#897 (comment)

I was thinking #21450 may mitigate this issue, but it would be nice to solve the issue and eliminate these fields completely.

---

Looks like the above example still reproduces on Dart 1.24.3, with some tweaks to prevent the Foo class from being fully optimized out:

main() {
  var foo = new Foo();
  print('$foo ${foo.hello}');
}

class Bar {
  String txt = " World";
}

class Foo {
  String _hello;
  Bar _world; // never initialized
  String get hello {
    if (_hello == null) {
      _hello = "Hello";
    }
    return _hello;
  }
  String get world {
    if (_world == null) {
      _world = new Bar();
    }
    return _world.txt;
  }
}

dart2js output

["", "reduced.dart",, T, {
  "^": "",
    main: function() {
    H.printString(H.Primitives_objectToHumanReadableString(new T.Foo(null, null)) + " hello");
  },
  Foo: {
    "^": "Object;_hello,_world"
   }
}, 1]

dart2js constructor retrieved by stepping into it at runtime

function Foo(p__hello, p__world) {
this._hello = p__hello;
this._world = p__world;
}
Foo.builtin$cls="Foo";
$desc=$collectedClasses.Foo[1];
Foo.prototype = $desc;

[yjbanov]

/cc @matanlurey who has been looking into tree-shaking issues not long ago

[Markzipan]

These examples no longe reproduce in Dart2JS. Closing