DDC source map hover info is often missing

[jmesserly]

Hover on many expressions does not work correctly:

var _baz = new Bar();

class Bar {
  int id = 42;
  String _name = "bar";
}

class Foo {
  bool _flag = false; // field names should be hoverable
  Bar _bar = new Bar();
  List<Bar> _bars = [new Bar(), new Bar()];

  bool get check => _flag && _bar.id == 42; // _flag points at `dart.test` helper

  void dump() {
    _flag = true;
    var x = _flag ? _bar.id : this._bar.id; // this._bar doesn't work, shows full expr value
    var z = _baz.id;
    var self = this;
    var w = self._bar.id; // self._bar doesn't work
    var name = _bar._name;
    print(this._bars[1]); // indexing doesn't work (not clear if we can fix)
    print(check);
    print(x + z);
  }
}

void main() {
  var foo = new Foo();
  foo.dump();
}

[jmesserly]

I'm seeing less problems than we were previously, possibly due to this fix https://dart-review.googlesource.com/c/sdk/+/39860

[jmesserly]

https://dart-review.googlesource.com/c/sdk/+/42028 fixes many of these.

Lazy top-level/static fields are not fixed yet; those require us to emit additional code or comments so we can mark an expression to evaluate (e.g. we need to mark JS code like library_name._bar to be able to hover over top-level field _bar). Same for dynamic gets (dload).

EDIT: also array indexing, but that cannot be fixed under the current DDC+Chrome debugger heuristic. DDC has to emit a method call because Dart List<T> is an interface type and we don't know what operator [] will do (also for bounds checking on the built-in list type). Chrome doesn't want to evaluate method calls. So we're at an impasse on that one.