Add mock mixin for Polymer.IronFitBehavior

Polymer.IronFitBehavior doesn't have a mixin and should be added to
component using Polymer.mixinBehavior([Polymer.IronFitBehavior]).
Polymer linter doesn't process this correctly. To workaround it the mock
mixin was added. It will be used in JSDoc comments after conversion to
class-based elements.

Change-Id: I61be019129879efe50c11c0f7dd56d6cef679136

polygerrit-ui/app/types/polymer-behaviors.js

@@ -0,0 +1,52 @@
+/**
+ * @license
+ * Copyright (C) 2019 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.
+ */
+
+/**
+ * For the purposes of template type checking, externs should be added for
+ * anything set on the window object. Note that sub-properties of these
+ * declared properties are considered something separate.
+ *
+ * This file is only for template type checking, not used in Gerrit code.
+ */
+
+/* eslint-disable no-var */
+/* eslint-disable no-unused-vars */
+
+function PolymerMixins() {
+  // This function must not be called.
+  // Due to an issue in polymer linter the linter can't
+  // process correctly some behaviors from Polymer library.
+  // To workaround this issue, here we define a minimal mixin to allow
+  // linter process our code correctly. You can add more properties to mixins
+  // if needed.
+
+  // Important! Use mixins from these file only inside JSDoc comments.
+  // Do not use it in the real code
+
+  /**
+   * @polymer
+   * @mixinFunction
+   * */
+  Polymer.IronFitMixin = base =>
+    class extends base {
+      static get properties() {
+        return {
+          positionTarget: Object,
+        };
+      }
+    };
+}