title: Polymer 3 build - bundle errors in browser when trying to access mixin
body: <p>I have a Polymer 3 app that works with no issues. But if I build and bundle it I am having issues when bundled. I am only doing es6 for now. I have no idea how to even go about trouble shooting this..pretty stuck.</p>
  
  <p>Polymer.json</p>
  
  <pre><code>  "builds": [
      {
        "name": "es6-bundled",
        "browserCapabilities": [
          "es2015"
        ],
        "js": {
          "minify": false,
          "transformModulesToAmd": true
        },
        "css": {
          "minify": false
        },
        "html": {
          "minify": false
        },
        "bundle": true,
        "addServiceWorker": false
      }
    ],
  </code></pre>
  
  <hr>
  
  <p>Error in browser:</p>
  
  <pre><code>all-items:formatted:211 Uncaught TypeError: (0 , _mainApp.PauseForRipple) is not a function
      at user-login.js? [sm]:4
  </code></pre>
  
  <hr>
  
  <p>Code from bundle:</p>
  
  <pre><code>define(["../main-app.js"], function(_mainApp) {
      "use strict";
  
      class UserLogin extends (0,
      _mainApp.PauseForRipple)(_mainApp.PolymerElement) {
          static get template() {
              return _mainApp.html`
        &lt;style&gt;
          :host {
            display: flex;
            align-items: center;
  </code></pre>
  
  <hr>
  
  <p>Original code before build:</p>
  
  <pre><code>import { PauseForRipple } from '../mixins/pause-for-ripple-mixin.js';
  
  class UserLogin extends PauseForRipple(PolymerElement) {
    static get template() {
      return html`
        &lt;style&gt;
          :host {
            display: flex;
            align-items: center;
            justify-content: center;
  </code></pre>