feat(checkbox): add feature targeting for styles

[mmalerba]

This PR adds Sass feature targeting to the checkbox component as part of #4227.

Currently blocked on #4228 which adds the mdc-feature-targeting package.

[mmalerba]

FYI: @kfranqueiro

[codecov-io]

Codecov Report

Merging #4258 into master will increase coverage by 0.33%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4258      +/-   ##
==========================================
+ Coverage   98.58%   98.91%   +0.33%     
==========================================
  Files         127      127              
  Lines        5705     5705              
  Branches      762      762              
==========================================
+ Hits         5624     5643      +19     
+ Misses         81       62      -19

Impacted Files	Coverage Δ
packages/mdc-checkbox/index.js	100% <0%> (+1.2%)	⬆️
packages/mdc-ripple/index.js	100% <0%> (+2.08%)	⬆️
packages/mdc-auto-init/index.js	100% <0%> (+2.77%)	⬆️
packages/mdc-tab/index.js	96.49% <0%> (+3.5%)	⬆️
packages/mdc-ripple/util.js	100% <0%> (+3.5%)	⬆️
packages/mdc-base/component.js	100% <0%> (+4.16%)	⬆️
packages/mdc-switch/index.js	97.95% <0%> (+6.12%)	⬆️
packages/mdc-tabs/tab/index.js	100% <0%> (+9.09%)	⬆️
packages/mdc-radio/index.js	97.43% <0%> (+12.82%)	⬆️

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[kfranqueiro]

Thanks for sending this out so quick! I saw what you meant RE the potential for reordering a few styles to combine blocks and those mostly seem reasonable. I left comments on the ones I saw, along with a few other things.

[mmalerba]

I should point out that one change I did make was to remove this @at-root. It didn't seem necessary, and it would prevent people from doing something like:

@include mdc-checkbox();

.my-app-alternate-theme {
  // ... change some theme variables ...
  @include mdc-checkbox(color);
}

Correct me if I'm wrong though.

[kfranqueiro]

Yeah, I noticed this, and realized that it seemed unnecessary, since it wasn't nested under anything and afaik @include can be done top-level anyway. We might've gotten a little carried away when we were trying to reduce specificity.

[kfranqueiro]

LGTM, diffed before/after and the only diffs are moved properties which look fair. Currently running the mdc-checkbox screenshot tests locally to confirm no diffs, since they can't run on PRs from forks.

[mmalerba]

Oh interesting about the fork thing, should I be writing code on branches instead?

[kfranqueiro]

I presume we'd have to give you access to do that, right? I don't think it's too big of a deal in these cases to run a subset of our screenshot tests manually.

Here's the report with 0 diffs for the checkbox screenshot tests: https://storage.googleapis.com/mdc-web-screenshot-tests/kfranqueiro/2019/01/24/21_26_52_748/report/report.html?utm_source=cli&utm_medium=test_results