Add typed return value from functions

[sparhami]

We should add type information to returned elements where possible using goog.dom.TagName. For example, this might look like:

/**
 * @param {!goog.dom.TagName<T>|string} tagName
 * @return R
 * @template T
 * @template R := cond(isUnknown(T), 'Element', T) =:
 */
function open(tagName) {

}

This is may be more useful if we create bound element functions, where we can return the typed element in the callback. For example:

const TagName = goog.dom.TagName;
const div = function(config, cb) {
  open(TagName.DIV);
    cb(currentParent());
  close(TagName.DIV);
}

The usage would be:

div({...}, (el) => {
  // compiler knows el is an HTMLDivElement
});