Trying to get juice to work...

TestingJuicy

<link rel="import" href="/bower_components/polymer/polymer.html">
<link rel="import" href="/bower_components/juicy-tile-list/src/juicy-tile-list.html">

<link rel="import" href="/aos-elements/aos-content/aos-content.html">
<link rel="import" href="/aos-elements/aos-content-item/aos-content-item.html">
<link rel="import" href="/aos-elements/aos-packary/aos-packary.html">
<polymer-element name="aos-content-list">
<template>
<link rel="stylesheet" href="/css/styleguide.css">

<style>

    aos-content-item {
      background-color: white;
      padding: 25px;
      margin: 20px;
      display: block;
	}

</style>

<!-- Get json feed -->
<aos-content items="{{items}}" id="service"></aos-content>

      <juicy-tile-list>
      <template repeat="{{item in items}}">	
      	<aos-content-item titel="{{item.title}}" tekst="{{item.snippets[1].body}}" class="item">
        </aos-content-item> 
      </template>
      </juicy-tile-list>
      <script type="text/javascript">
        model = {
            items:[
            {
                desc: "first",
                attr: "a"
            },
            {
                desc: "second",
                attr: "b"
            },
            {
                desc: "third",
                attr: "c"
            },
            {
                desc: "forth",
                attr: "d"
            }
            ]
        };
        document.addEventListener("polymer-ready", function(){
            document.querySelector("template").model = model;
            
        });
    </script>
</template>


<script>
Polymer('aos-content-list', {
	
});
</script>
</polymer-element>

I'm not entirely sure what your aos- elements are supposed to do. Here https://gist.github.com/tomalec/d9107cc91b8bb71a93ec you have an example how to use juicy-tile-list inside another custom element.
Probably what you missed is that, juicy-tile-list is display: inline-block; by default so you need to give some initial dimension constraint to pack. So one of the solution could be to set juicy-tile-list { width: 100% }.