dartdoc generates broken links for inherited fields in enums

[jcollins-g]

As part of fixing #1360 I found this:

$ grind check-links
grinder running [check-links]

[check-links]
    * Not found: testing/test_package_docs/ex/Animal/hashCode.html from ex/Animal-class.html
    * Not found: testing/test_package_docs/ex/Animal/runtimeType.html from ex/Animal-class.html
    * Not found: testing/test_package_docs/ex/Animal/operator_equals.html from ex/Animal-class.html
    * Not found: testing/test_package_docs/ex/Animal/noSuchMethod.html from ex/Animal-class.html
    * Not found: testing/test_package_docs/ex/Animal/toString.html from ex/Animal-class.html
    * Not found: testing/test_package_docs/fake/Color/hashCode.html from fake/Color-class.html
    * Not found: testing/test_package_docs/fake/Color/runtimeType.html from fake/Color-class.html
    * Not found: testing/test_package_docs/fake/Color/operator_equals.html from fake/Color-class.html
    * Not found: testing/test_package_docs/fake/Color/noSuchMethod.html from fake/Color-class.html
    * Not found: testing/test_package_docs/fake/Color/toString.html from fake/Color-class.html

[jcollins-g]

Cause of the link breakage seems to be that we're just not generating the files when we are supposed to be.

[jcollins-g]

This has been broken since at least 0.9.11. Maybe just noone has noticed.

[jcollins-g]

Since this only impacts enums and was introduced in 0.9.11, no need to block releases on it.