Import Polymer 0.8 and several key elements.

third_party/polymer/v0_8/components/paper-input/paper-input-error.html

+<!--
+@license
+Copyright (c) 2015 The Polymer Project Authors. All rights reserved.
+This code may only be used under the BSD style license found at http://polymer.github.io/LICENSE.txt
+The complete set of authors may be found at http://polymer.github.io/AUTHORS.txt
+The complete set of contributors may be found at http://polymer.github.io/CONTRIBUTORS.txt
+Code distributed by Google as part of the polymer project is also
+subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
+-->
+
+<link rel="import" href="../polymer/polymer.html">
+<link rel="import" href="../paper-styles/paper-styles.html">
+
+<!--
+`<paper-input-error>` is an error message for use with `<paper-input-container>`.
+-->
+<dom-module id="paper-input-error">
+
+  <style>
+
+    :host {
+      /* need to use display: none for role="alert" */
+      display: none;
+      float: left;
+
+      color: var(--paper-input-container-invalid-color);
+
+      mixin(--paper-input-container-add-on);
+      mixin(--paper-input-container-add-on-font);
+
+      mixin(--paper-input-error);
+    }
+
+    :host([invalid]) {
+      display: inline-block;
+    };
+
+  </style>
+
+  <template>
+
+    <content></content>
+
+  </template>
+
+</dom-module>
+
+<script>
+
+(function() {
+
+  Polymer({
+
+    is: 'paper-input-error',
+
+    enableCustomStyleProperties: true,
+
+    hostAttributes: {
+      'add-on': '',
+      'role': 'alert'
+    },
+
+    properties: {
+
+      /**
+       * Set to true to show the error.
+       */
+      invalid: {
+        reflectToAttribute: true,
+        type: Boolean
+      }
+
+    },
+
+    attached: function() {
+      this.fire('addon-attached');
+    }
+
+  })
+
+})();
+
+</script>