Add mock mixins to pass polymer linter checks

Polymer linter doesn't process Polymer.mixinBehaviors(...) correctly.
This will lead to a problem after swtiching to a class-based components. To
workaround these issue, the mock mixins are added as a temporary
solution. After converting to a class-based element, all behaviors will
be converted to mixin classes.

Change-Id: I8dc3cb9767be3aa08c67fedbff77105e4b8b4ec1
diff --git a/polygerrit-ui/app/behaviors/gr-repo-plugin-config-behavior/gr-repo-plugin-config-behavior.html b/polygerrit-ui/app/behaviors/gr-repo-plugin-config-behavior/gr-repo-plugin-config-behavior.html
index 2fa9191..69ebf23 100644
--- a/polygerrit-ui/app/behaviors/gr-repo-plugin-config-behavior/gr-repo-plugin-config-behavior.html
+++ b/polygerrit-ui/app/behaviors/gr-repo-plugin-config-behavior/gr-repo-plugin-config-behavior.html
@@ -34,5 +34,22 @@
     },
     PLUGIN_CONFIG_CHANGED: 'plugin-config-changed',
   };
+
+  // eslint-disable-next-line no-unused-vars
+  function defineEmptyMixin() {
+    // This is a temporary function.
+    // Polymer linter doesn't process correctly the following code:
+    // class MyElement extends Polymer.mixinBehaviors([legacyBehaviors], ...) {...}
+    // To workaround this issue, the mock mixin is declared in this method.
+    // In the following changes, legacy behaviors will be converted to mixins.
+
+    /**
+     * @polymer
+     * @mixinFunction
+     */
+    Gerrit.RepoPluginConfigMixin = base =>
+      class extends base {
+      };
+  }
 })(window);
 </script>