title: ESLint imports/modules are treated as undefined
body: <p>I'm using Webpack with Polymer, importing a component like so:</p>
  
  <pre><code>import '@polymer/polymer/polymer-element.html';
  
  class AppShell extends Polymer.Element {
    static get is() { return 'app-shell'; }
  }
  </code></pre>
  
  <p>I left out the rest of the component here. The import is working as expected in my app, but when I am running ESLint, I get the following error message:</p>
  
  <pre><code>50:27  error  'Polymer' is not defined  no-undef
  </code></pre>
  
  <p>This is my <code>package.json</code>, where I'm defining my ESLint settings. Anyone have an idea why ESLint is not picking up the import properly?</p>
  
  <pre><code>{
    "name": "client-meeting-tracker",
    "version": "1.0.0",
    "description": "",
    "main": "index.js",
    "private": true,
    "scripts": {
      "start": "node utils/webserver.js",
      "lint": "eslint --ext .html,.js */**",
      "build": "node utils/build.js"
    },
    "dependencies": {
      "@startup-boilerplate/inkling": "*",
      "auth0-lock": "^10.7.3",
      "lodash": "^4.6.1",
      "moment": "^2.17.1"
    },
    "devDependencies": {
      "cross-env": "^1.0.6",
      "css-loader": "^0.23.1",
      "eslint": "^3.16.0",
      "eslint-plugin-html": "^2.0.1",
      "eslint-plugin-import": "^2.2.0",
      "file-loader": "^0.8.4",
      "fs-extra": "^0.30.0",
      "polymer-cli": "^0.17.0",
      "wc-loader": "*",
      "webpack": "2.2.0",
      "webpack-dev-server": "2.2.0"
    },
    "eslintConfig": {
      "env": {
        "browser": true,
        "commonjs": true,
        "es6": true
      },
      "extends": [
        "eslint:recommended"
      ],
      "parserOptions": {
        "sourceType": "module"
      },
      "rules": {
        "indent": [
          "error",
          "tab",
          {
            "SwitchCase": 1
          }
        ],
        "quotes": [
          "error",
          "single"
        ],
        "semi": [
          "error",
          "always"
        ]
      },
      "plugins": [
        "html"
      ]
    }
  }
  </code></pre>