Add Object.assign to list of required polyfills

[jimmydief]

Fixes #1431.

Pull Request Review checklist (do not remove)

• npm run lint:fix has been run and linting passes.
• Mocha, Jest (Storyshots), and components/component-docs.json CI tests pass (npm test).
• Tests have been added for new props to prevent regressions in the future. See readme.
• Review the appropriate Storybook stories. Open http://localhost:9001/.
• The Accessibility panel of each Storybook story has 0 violations (aXe). Open http://localhost:9001/.
• Review tests are passing in the browser. Open http://localhost:8001/.
• Review markup conforms to SLDS by looking at DOM snapshot strings.
• Add year-first date and commit SHA to last-slds-markup-review in package.json and push.
• Request a review of the deployed Heroku app by the Salesforce UX Accessibility Team.
• Add year-first review date, and commit SHA, last-accessibility-review, to package.json and push.
• While the contributor's branch is checked out, run npm run local-update within locally cloned site repo to confirm the site will function correctly at the next release.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @jimmydief to sign the Salesforce.com Contributor License Agreement.

[interactivellama]

Thanks. If you ever see one and want to change it to an ES6 object spread .... Babel will polyfill it for you.