Transitive linkPaths broken in 2.0

[alan-strohm]

Description

Calling linkPaths(a, b) and then linkPaths(b, c) doesn't link c to a.

Live Demo

In 1.0 the checkbox is unchecked:
http://jsbin.com/hocuwazada/edit?html,output

Same code, but with 2.0 the checkbox stays checked:
http://jsbin.com/xevedoyeme/edit?html,output

Steps to Reproduce

       this._foo2 = this._foo;
        this._foo3 = this._foo;
        this.linkPaths('_foo', '_foo2');
        this.linkPaths('_foo2', '_foo3')
        this.set('_foo3.checked', false);

Expected Results

A notification is triggered on _foo.

Actual Results

A notification is triggered on _foo2 but not _foo.

Browsers Affected

• [ X] Chrome
• Firefox
• Edge
• Safari 9
• Safari 8
• IE 11

Versions

• Polymer: v2.0-preview
• webcomponents: v2.0-preview