<core-transition-css>` implements CSS transitions as `<core-transition>` objects so they can be
reused in a pluggable transition system such as in `<core-overlay>`. Currently this class has
some specific support to animate an element from and to the viewport such as a dialog, but you
can override it for different effects.

Example:

my-css-transition.html:

    <polymer-element name="my-css-transition" extends="core-transition-css">
    <template>
        <style>
            :host(.my-transition) {
        