Shady DOM doesn't correctly parse custom property rules.

[philipwalton]

Consider the following element:

<dom-module id="my-element">
  <style>
    :host {
      x--color: red;
    }
    p {
      color: var(--color);
    }
  </style>
  <template>
    <div>
      <p>Some paragraph text...</p>
    </div>
  </template>
</dom-module>

<script>
  Polymer({
    is: "my-element",

    // The custom properties shim is currently an opt-in feature
    enableCustomStyleProperties: true
  });
</script>

<my-element></my-element>

The text should not be red because x--color: red is not a valid property/value combination, but in Chrome it is red.