Use es6 methods instead of fakejquery.

In particular, use `Array.isArray` instead of `$.isArray`, and
`Object.assign` instead of `$.extend`.  `Object.assign` operates only on own
properties, so use `Object.create` on the prototype where necessary to
get inherited properties.  `Object.assign` does a simple assignment and
is appropriate in most places, but be careful if we ever install
getters/setters on a prototype.

Implement `Util.clone()` from scratch to have a jquery-compatible deep
clone operation.  In particular, this needs to ignore objects which
aren't "plain objects", so we don't try to clone DOM nodes.  Our
definition of a "plain object" is compatible with jquery/zepto.js, and
is thus something of a hack.  We should eventually replace this with a
`console.assert()` and remove/rewrite the places where we try to clone
objects which contain DOM trees and other cruft.

Change-Id: I88c8fe41a9be84c167d5a0ea1187fd258f077968