Restore createAssigner (fixes compilation of merge and mergeWith) (#4101)

.internal/isIterateeCall.js

@@ -0,0 +1,29 @@
+import isArrayLike from '../isArrayLike.js'
+import isIndex from './isIndex.js'
+
+/**
+ * Checks if the given arguments are from an iteratee call.
+ *
+ * @private
+ * @param {*} value The potential iteratee value argument.
+ * @param {*} index The potential iteratee index or key argument.
+ * @param {*} object The potential iteratee object argument.
+ * @returns {boolean} Returns `true` if the arguments are from an iteratee call,
+ *  else `false`.
+ */
+
+function isIterateeCall(value, index, object) {
+  if (!isObject(object)) {
+    return false;
+  }
+  const type = typeof index;
+  if (type == 'number'
+        ? (isArrayLike(object) && isIndex(index, object.length))
+        : (type == 'string' && index in object)
+      ) {
+    return eq(object[index], value);
+  }
+  return false;
+}
+
+export default isIterateeCall