eslint: no-unused-vars incorrectly raised when variable is used with for…in

The no-unused-vars error is incorrectly triggered when a variable is only used in a for in statement.

The following snippet gives the error no-unused-vars:

(function () {
  "use strict";
  var key;
  for (key in this) return;
})();

3:6  error  key is defined but never used  no-unused-vars

However, the variable is clearly used in for (key in this). Furthermore, omitting the declaration also results in an error:

(function () {
  "use strict";
  for (key in this) return;
})();

3:7  error  "key" is not defined  no-undef

The expected behavior is that the first snippet does not yield any error; the second error is correct.

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