Revert "chore: Remove unused isFocusable*() functions."

This reverts commit 404c20eeaf.

core/interfaces/i_focusable_node.ts

@@ -60,3 +60,20 @@ export interface IFocusableNode {
    */
   onNodeBlur(): void;
 }
+
+/**
+ * Determines whether the provided object fulfills the contract of
+ * IFocusableNode.
+ *
+ * @param object The object to test.
+ * @returns Whether the provided object can be used as an IFocusableNode.
+ */
+export function isFocusableNode(object: any | null): object is IFocusableNode {
+  return (
+    object &&
+    'getFocusableElement' in object &&
+    'getFocusableTree' in object &&
+    'onNodeFocus' in object &&
+    'onNodeBlur' in object
+  );
+}

core/interfaces/i_focusable_tree.ts

@@ -119,3 +119,22 @@ export interface IFocusableTree {
    */
   onTreeBlur(nextTree: IFocusableTree | null): void;
 }
+
+/**
+ * Determines whether the provided object fulfills the contract of
+ * IFocusableTree.
+ *
+ * @param object The object to test.
+ * @returns Whether the provided object can be used as an IFocusableTree.
+ */
+export function isFocusableTree(object: any | null): object is IFocusableTree {
+  return (
+    object &&
+    'getRootFocusableNode' in object &&
+    'getRestoredFocusableNode' in object &&
+    'getNestedTrees' in object &&
+    'lookUpFocusableNode' in object &&
+    'onTreeFocus' in object &&
+    'onTreeBlur' in object
+  );
+}