Move commons into /app (#2486)

* stash

* move commons into the app

packages/app/lib/common/ReferenceBase.js

@@ -0,0 +1,43 @@
+/*
+ * Copyright (c) 2016-present Invertase Limited & Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this library 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.
+ *
+ */
+
+export default class ReferenceBase {
+  constructor(_path) {
+    let path = _path;
+
+    if (path) {
+      path = path.length > 1 && path.endsWith('/') ? path.substring(0, path.length - 1) : path;
+      if (path.startsWith('/') && path.length > 1) {
+        path = path.substring(1, path.length);
+      }
+    } else {
+      path = '/';
+    }
+
+    this.path = path;
+  }
+
+  /**
+   * The last part of a Reference's path (after the last '/')
+   * The key of a root Reference is null.
+   * @type {String}
+   * {@link https://firebase.google.com/docs/reference/js/firebase.database.Reference#key}
+   */
+  get key() {
+    return this.path === '/' ? null : this.path.substring(this.path.lastIndexOf('/') + 1);
+  }
+}