Initiates observation and returns the initial value. The callback will be passed the updated value, and may optionally be declared to take a second argument, which will contain the previous value.
Implementation note: a third argument can also be declared, which will
receive a list of objects and paths, such that list[2 * i] will access
the object and list[2 * i + 1] will access the path, where i is the
order of the addPath call. This parameter is only used by
package:polymer as a performance optimization, and should not be relied
on in new code.
open(callback) => super.open(callback);