Polymer 2.0 getElementById in different ShadowDom

I'm trying to learn Polymer 2.0 but am stuck with the problem of getting an element from a different Shadow Dom. This worked in Polymer 1 but no longer in Polymer 2.0. What is the correct way of writing this? It just tells me that the targetText = Null.

Thanks for your help!

This is a MWE: Polymer WC 1:

<dom-module id="sc-navdrawer">
<template>
<style is="custom-style">
p {
  font-weight: bold;
}
.changed {
  color: red;
}
</style>
<p>Some text in normal p tag</p>
<div id="test" class="htmltextcontent" inner-h-t-m-l="{{inputText}}"></div>
</template>

<script>
    Polymer({
        is: 'sc-navdrawer',
        properties: {
          inputText: {
            type: String,
            value: "<p>Some innerhtml text in p tags</p>"
          }
        }
    });

</script>
</dom-module>

Polymer WC 2:

<dom-module id="sc-testpage">
<template>

<button onclick="{{changeColor}}">Click here to change color</button>
</template>

<script>
    Polymer({
        is: 'sc-testpage',
        changeColor: function() {
          var targetText = document.getElementById("test");
          console.log(targetText);
          targetText.classList.add("changed");
        }
    });

</script>
</dom-module>