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diff --git a/polygerrit-ui/app/behaviors/safe-types-behavior/safe-types-behavior.html b/polygerrit-ui/app/behaviors/safe-types-behavior/safe-types-behavior.html
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+<!--
+@license
+Copyright (C) 2018 The Android Open Source Project
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+-->
+<script>
+(function(window) {
+  'use strict';
+
+  window.Gerrit = window.Gerrit || {};
+
+  /** @polymerBehavior Gerrit.SafeTypes */
+  Gerrit.SafeTypes = {};
+
+  const SAFE_URL_PATTERN = /^(https?:\/\/|mailto:|\/|#)/i;
+
+  /**
+   * Wraps a string to be used as a URL. An error is thrown if the string cannot
+   * be considered safe.
+   * @constructor
+   * @param {string} url the unwrapped, potentially unsafe URL.
+   */
+  Gerrit.SafeTypes.SafeUrl = function(url) {
+    if (!SAFE_URL_PATTERN.test(url)) {
+      throw new Error(`URL not marked as safe: ${url}`);
+    }
+    this._url = url;
+  };
+
+  /**
+   * Get the string representation of the safe URL.
+   * @returns {string}
+   */
+  Gerrit.SafeTypes.SafeUrl.prototype.asString = function() {
+    return this._url;
+  };
+
+  Gerrit.SafeTypes.safeTypesBridge = function(value, type) {
+    // If the value is being bound to a URL, ensure the value is wrapped in the
+    // SafeUrl type first. If the URL is not safe, allow the SafeUrl constructor
+    // to surface the error.
+    if (type === 'URL') {
+      let safeValue = null;
+      if (value instanceof Gerrit.SafeTypes.SafeUrl) {
+        safeValue = value;
+      } else if (typeof value === 'string') {
+        safeValue = new Gerrit.SafeTypes.SafeUrl(value);
+      }
+      if (safeValue) {
+        return safeValue.asString();
+      }
+    }
+
+    // If the value is being bound to a string or a constant, then the string
+    // can be used as is.
+    if (type === 'STRING' || type === 'CONSTANT') {
+      return value;
+    }
+
+    // Otherwise fail.
+    throw new Error(`Refused to bind value as ${type}: ${value}`);
+  };
+})(window);
+</script>