dartdoc integration test needs to have a mode that doesn't fail on trivial doc changes

[jcollins-g]

To fix the test, I suggest adding a dartdoc command line option to drop inherited text from the SDK in generated docs and replace it with a stub. This way we can still verify that dartdoc is generating the right structure and is choosing correctly when to import docs from the SDK without depending on the exact contents of those docs themselves.

I believe this is the main blocker on running our tests as part of the main suite in the SDK.

@devoncarew