Import Polymer 0.8 and several key elements.

third_party/polymer/v0_8/components-chromium/paper-input/paper-input-error-extracted.js

+
+
+(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');
+    }
+
+  })
+
+})();
+