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