* Adds a new extension point to allow for contributing OptionsValidators.
* Adds a new linter 'service' to server that contributes a lint-aware validator.
You'll notice that the warning code I added is NOT in `errors.dart`. I did that in anticipation of plugin-specifc warnings living elsewhere. If that's not the right angle, happy to un-do!
R=brianwilkerson@google.com
Review URL: https://codereview.chromium.org/1418333002 .
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