Fix most current canonical score lints

[parlough]

You can find the current set here: https://github.com/dart-lang/linter/blob/master/tool/canonical/score.yaml

I disabled non_constant_identifier_names for now due to how our tests are written, but I did fix a few cases of this line.

This PR also deprecates two members.

HTMLBASE_PLACEHOLDER - Replaced with htmlBasePlaceholder which is marked as @internal. This is due to the old constant naming scheme as well as it doesn't seem like this member(or the behavior it enables) should be used externally.

PackageGraph.UninitializedPackageGraph -> Replaced with PackageGraph.uninitialized to fit the appropriate naming style and since the second PackageGraph is unnecessary.

[coveralls]

Coverage decreased (-0.03%) to 91.611% when pulling c7d74cf on parlough:fix/canonical-score-lints into 5b6a529 on dart-lang:master.

[jcollins-g]

What is the motivation for using this particular set of lints? We're mostly going off of pedantic.

[parlough]

You can find some more information here. This is the pub scoring set, which @devoncarew described a derivative of as "a potential proposal for a new recommended rule set for Dart". While the dartdoc package isn't commonly depended on, it will be nice to be consistent with other packages. I don't know much about the specific details as it seems to perhaps be discussed more internally, but I still see it as a win. There is also a recommended set, but that one is larger and can be explored later on.

Even if the final recommendations change, I thought it would be good if we followed them earlier, so future code already follows it and eases the transition.

[jcollins-g]

Yes, this does seem to be the way of the future, thanks for getting ahead of it!

Only one change needed; the addition of a hashCode calculation on Class could result in problems.