I wanted to resolve #9, so did some digging on the lint failure. It's been a while since we responded to the PR last so I figured I'd take on doing the rest, instead of prompting the author to update their PR. 😬

• Upgrade Flow to fix the type error, where shadowRoot does not have getElementById. It was added in facebook/flow@72b6397.
• Add test for shadowRoot context. Before this patch:

FAILED TESTS:
  clipboard-copy element
    shadow DOM context
      ✖ node
        HeadlessChrome 78.0.3904 (Mac OS X 10.14.6)
      AssertionError: expected 'Target in Document' to equal 'Target in shadowRoot'
          at test.js:216:16

cc @jorgecasar