UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/54.0.2840.71 Safari/537.36

Steps to reproduce the problem:
1. Make custom element:

    <!-- ELEMENT -->
    <div class='outer'>
        <div class='inner'>

            <!-- FIRST ELEMENT -->
            <slot name='x-first'></slot>

            <!-- ANY ELEMENT -->
            <slot></slot>

            <!-- LAST ELEMENT -->
            <slot name='x-last'></slot>

        </div>
    </div>

2. Define custom component: customElements.define('x-element',  xElement);

3. Use custom component <x-element>: 

    <x-element>

        <!-- FIRST ELEMENT -->
        <x-first slot='x-first'> custom element 1 </x-first>

        <!-- ANY ELEMENT -->
        see expected behaviour

        <!-- LAST ELEMENT -->
        <x-last slot='x-last'>custom element 3</x-last>

    </x-element>

What is the expected behavior?
Possibility to put any element regardless of it's origin, i.e. <div> or <img> or custom element <x-something> 

What went wrong?
<slot></slot> ignores custom element. 

    <x-element>

        <!-- FIRST ELEMENT -->
        <x-first slot='x-first'> custom element 1 </x-first>

        <!-- ANY ELEMENT -->
        <x-new slot='x-new'> </x-new>

        <!-- LAST ELEMENT -->
        <x-last slot='x-last'>custom element 3</x-last>

    </x-element>

Custom element like <x-new slot='x-new'> does not appear. 
Meanwhile, in Firefox result as expected. 
For some reason in G-chrome ignores custom element.

Did this work before? N/A 

Does this work in other browsers? Yes

Chrome version: 54.0.2840.71  Channel: n/a
OS Version: OS X 10.11.6
Flash Version: Shockwave Flash 24.0 r0

To my understanding the <slot></slot> element that does not have attribute 'name'  should in theory put ANY content submitted to it.