Refactor: Break Apart Component Mutation Observer Logic

[sghoweri]

Jira

N/A

Summary

Slightly refactors + breaks out the mutation observer logic currently used in the button component into it's own standalone JS utility library for reuse in Bolt core.

How to test

Confirm button components rendering to the Shadow DOM re-render as expected when slotted content added (including the correct slot classes change accordingly).

Create a new button on the page and confirm the before / after works as expected.

const buttonIconBefore = document.createElement('bolt-icon');
buttonIconBefore.setAttribute('name', 'arrow-left');
buttonIconBefore.setAttribute('slot', 'before');

const buttonIconAfter = document.createElement('bolt-icon');
buttonIconAfter.setAttribute('name', 'chevron-right');
buttonIconAfter.setAttribute('slot', 'after');

const buttonElement = document.querySelector('bolt-button'); 
buttonElement.textContent = 'Hello world!';
buttonElement.appendChild(buttonIconAfter);

document.body.appendChild(buttonElement); // confirm the before slot has the `is-empty` CSS class.

buttonElement.appendChild(buttonIconBefore); // now confirm the before slot contains the new icon and no longer displays the `is-empty` CSS class.

---

Screenshot before the "before" slot is populated with content

Screenshot after the "before" slot is populated with content

[remydenton]

Any idea why this PR didn't get a now.sh build @sghoweri?

[sghoweri]

Any idea why this PR didn't get a now.sh build @sghoweri?

@remydenton huh... that's weird!

[sghoweri]

@remydenton looks like this could either be:

1. a random now.sh issue OR

2. A random now.sh issue... possibly related to the number of concurrent builds / deploys we have going on? I'm curious to see if this problem goes away if we upgrade our now.sh plan to the next tier... particularly since I've seen this similar issue pop up a number of times now.

[sghoweri]

@remydenton working URL I pulled from the Travis logs: https://boltdesignsystem-vlrooyzhtk.now.sh

[remydenton]

Looks great. Thanks for the testing instructions!