Rolling forward https://codereview.chromium.org/1155683008/ which was

third_party/polymer/v0_8/components-chromium/more-routing/emitter-extracted.js

-
-(function(scope) {
-var MoreRouting = scope.MoreRouting = scope.MoreRouting || {};
-MoreRouting.Emitter = Object.create(null);  // Minimal set of properties.
-
-/**
- * A dumb prototype that provides very simple event subscription.
- *
- * You are responsible for initializing `__listeners` as an array on objects
- * that make use of this.
- */
-Object.defineProperties(MoreRouting.Emitter, {
-
-  /**
-   * Registers a callback that will be called each time any parameter managed by
-   * this object (or its parents) have changed.
-   *
-   * @param {!Function} callback
-   * @return {{close: function()}}
-   */
-  __subscribe: {
-    value: function __subscribe(callback) {
-      this.__listeners.push(callback);
-
-      return {
-        close: this.__unsubscribe.bind(this, callback),
-      };
-    },
-  },
-
-  /**
-   * Unsubscribes a previously registered callback.
-   *
-   * @param {!Function} callback
-   */
-  __unsubscribe: {
-    value: function __unsubscribe(callback) {
-      var index = this.__listeners.indexOf(callback);
-      if (index < 0) {
-        console.warn(this, 'attempted unsubscribe of unregistered listener:', callback);
-        return;
-      }
-      this.__listeners.splice(index, 1);
-    },
-  },
-
-  /**
-   * Notifies subscribed callbacks.
-   */
-  __notify: {
-    value: function __notify(key, value) {
-      if (this.__silent) return;
-      var args = Array.prototype.slice.call(arguments);
-      // Notify listeners on parents first.
-      var parent = Object.getPrototypeOf(this);
-      if (parent && parent.__notify && parent.__listeners) {
-        parent.__notify.apply(parent, args);
-      }
-
-      this.__listeners.forEach(function(listener) {
-        listener.apply(null, args);
-      }.bind(this));
-    },
-  },
-
-});
-
-})(window);