Move internal modules to “internal” folder.

.internal/baseFlatten.js

@@ -0,0 +1,38 @@
+import arrayPush from './.internal/arrayPush.js';
+import isFlattenable from './.internal/isFlattenable.js';
+
+/**
+ * The base implementation of `flatten` with support for restricting flattening.
+ *
+ * @private
+ * @param {Array} array The array to flatten.
+ * @param {number} depth The maximum recursion depth.
+ * @param {boolean} [predicate=isFlattenable] The function invoked per iteration.
+ * @param {boolean} [isStrict] Restrict to values that pass `predicate` checks.
+ * @param {Array} [result=[]] The initial result value.
+ * @returns {Array} Returns the new flattened array.
+ */
+function baseFlatten(array, depth, predicate, isStrict, result) {
+  let index = -1;
+  const length = array.length;
+
+  predicate || (predicate = isFlattenable);
+  result || (result = []);
+
+  while (++index < length) {
+    const value = array[index];
+    if (depth > 0 && predicate(value)) {
+      if (depth > 1) {
+        // Recursively flatten arrays (susceptible to call stack limits).
+        baseFlatten(value, depth - 1, predicate, isStrict, result);
+      } else {
+        arrayPush(result, value);
+      }
+    } else if (!isStrict) {
+      result[result.length] = value;
+    }
+  }
+  return result;
+}
+
+export default baseFlatten;