Get rid of the helpers {un,}wrapJavaScriptException.

They are replaced by proper Scala.js method in the
scala.scalajs.runtime package object.
js.Dynamic.newInstance returns Object with Dynamic.

Instead of just Dynamic.
Mark js.WrappedDict as inlineable, and override its keys method.
Get rid of the objectClone helper.

By implementing it in Scala.js in the scala.scalajs.runtime
package object.