For this PR, I only updated the button demo page to use the new demoReady() function. I will create a separate follow-up PR to update the remaining demo pages to use demoReady().

# DO NOT MERGE

This PR depends on #1919. Please merge #1919 before merging this PR.

Our packaged JS is now being loaded with `async`, which speeds up initial rendering time over slow connections. Using "Mid-tier mobile" network emulation in Chrome devtools, the [TTFMP](https://developers.google.com/web/tools/lighthouse/audits/first-meaningful-paint) of the button demo page was reduced from ~10.5s to ~9.5s, and the time to render the Material icon font dropped from ~12s to ~10.5s.

# Conflicts:
#	demos/button.html
#	demos/checkbox.html
#	demos/dialog.html
#	demos/drawer/persistent-drawer.html
#	demos/drawer/temporary-drawer.html
#	demos/fab.html
#	demos/grid-list.html
#	demos/icon-toggle.html
#	demos/linear-progress.html
#	demos/list.html
#	demos/radio.html
#	demos/ready.js
#	demos/ripple.html
#	demos/select.html
#	demos/simple-menu.html
#	demos/slider.html
#	demos/snackbar.html
#	demos/tabs.html
#	demos/text-field.html
#	demos/theme/index.html
#	demos/toolbar/default-flexible-toolbar.html
#	demos/toolbar/menu-toolbar.html
#	demos/toolbar/waterfall-flexible-toolbar-custom-style.html
#	demos/toolbar/waterfall-flexible-toolbar.html
#	demos/toolbar/waterfall-toolbar-fix-last-row.html
#	demos/toolbar/waterfall-toolbar.html

commit d62beff
Author: Andrew C. Dvorak <[email protected]>
Date:   Fri Jan 12 12:42:09 2018 -0800

    chore(demos): Remove ripple examples from theme page (#1954)

    Ripple isn't an actual component, and other components on the page already have ripples, so there's no need for separate ripple examples on the page.

commit 145217c
Author: Will Ernest <[email protected]>
Date:   Fri Jan 12 09:01:55 2018 -0800

    chore(select): Remove multi-select from mdc-select. (#1917)

    BREAKING CHANGE: Removes mdc-multi-select from the mdc-select package. Use lists to create components that allow multiple items to be selected.

# Conflicts:
#	demos/theme/index.html

commit 82a9fa3
Author: Will Ernest <[email protected]>
Date:   Fri Jan 12 14:18:37 2018 -0800

    fix(select): Remove unused JS logic for bottom-line scaleX transform (#1910)

    fix(select): Remove unused JS logic for bottom-line scaleX transform and matching test