[breaking change] Change return type of isTruthy and not

[srujzs]

Some more context here: #55238. This is the "reverse" of that change - move all operators to use JS types instead of returning Dart types.

Intended Change

dart:js_interop's isTruthy and not should now return a JSBoolean instead of a bool.

dart:js_interop exposes isTruthy and not operators to be consistent with dart:js_util API. They currently return a bool, as the result is always a boolean JS value.

Justification

JS operator utility members should use JS types instead of their Dart equivalents for performance. They are likely to be used when the conversion is deemed costly (otherwise, users could just convert to the Dart value and use Dart operators) and they are likely to be composed on one other e.g. b1.and(b2.or(b3)). In the case of composition, this adds several additional unneeded conversions.

Impact

Minimal. There are no usages in google3, in Flutter, in the SDK, or on GitHub.

Mitigation

Convert the resulting JSBoolean to a boolean with toDart:

bool b1 = true.toJS.not;
bool b2 = true.toJS.isTruthy;

bool b1 = true.toJS.not.toDart;
bool b2 = true.toJS.isTruthy.toDart;

cc @sigmundch

[sigmundch]

LGTM 👍, this would bring all operators to have a consistent API: all other operators today return a JS type, this would make the dart:js_interop library homogeneous.

cc @lrhn @kevmoo

cc @itsjustkevin - for the breaking change process

---

In terms of timing, I'm open to either having this in 3.4 or 3.5, given the cutoff date, I'm fine waiting until 3.5. Usage is low enough that we don't expect much impact later either, but we may want to suggest a backward compatible way to write code if we don't release it in 3.4. For example:

bool isReallyTruthy(JSAny a) {
  Object o = a.isThruthy;
  return o is bool ? o : (o as JSBoolean).toDart;
}

[itsjustkevin]

@vsmenon @Hixie @grouma for review.

[grouma]

SGTM.

[itsjustkevin]

Pinging @vsmenon and @Hixie

[Hixie]

no objection; seems reasonable but i don't have enough context to fully evaluate.

[vsmenon]

lgtm

[itsjustkevin]

Breaking change approved