error using mixins as computed property or in template methods in 0.8.0-rc.7 #1456

<link rel="import" href="bower_components/polymer/polymer.html">
<link rel="import" href="sample-element-mixin.html">

<dom-module id="sample-element">
  <link rel="stylesheet" type="css" href="sample-element.css">
  <template>
    <div>
      <section hidden$="{{hideApp}}">
        <div hidden$="{{!isChromeRuntime}}">
            <h1>Configuration</h1>
            <div>Hide App:</div><span>{{config.showDefaults.hideApp}}</span>
        </div>
      </section>
  </template>
</dom-module>
<script>
    Polymer({

      mixins:[SampleElementMixin],

      is:'sample-element',

      properties:{

        config:{
          type:Object,
          value:function(){ return {"showDefaults":{
            "hideApp":true
          }};}
        },
        hideApp:{
          type:Boolean,
          value:false
        },
        isChromeRuntime:{
          type:Boolean,
          computed:'computeIsChromeRuntime(config)',
          observer:'isChromeRuntimeChanged'
        }
      },
      created:function(){
        if(this._data)
          console.log("created",this.isChromeRuntime);
      },
      ready:function(){
        console.log("ready",this.isChromeRuntime);
      },
      attached:function(){
        console.log("attached",this.isChromeRuntime);
      },
      isChromeRuntimeChanged:function(val){
          console.log("changed",val);
      }
    });
</script>

....

<script>
  SampleElementMixin={

    computeIsChromeRuntime:function(config){
      return "chrome" in window && "runtime" in window.chrome;
    }

  }
</script>