Move internal modules to “internal” folder.

.internal/shortOut.js

@@ -0,0 +1,37 @@
+/** Used to detect hot functions by number of calls within a span of milliseconds. */
+const HOT_COUNT = 800;
+const HOT_SPAN = 16;
+
+/* Built-in method references for those with the same name as other `lodash` methods. */
+const nativeNow = Date.now;
+
+/**
+ * Creates a function that'll short out and invoke `identity` instead
+ * of `func` when it's called `HOT_COUNT` or more times in `HOT_SPAN`
+ * milliseconds.
+ *
+ * @private
+ * @param {Function} func The function to restrict.
+ * @returns {Function} Returns the new shortable function.
+ */
+function shortOut(func) {
+  let count = 0;
+  let lastCalled = 0;
+
+  return function(...args) {
+    const stamp = nativeNow();
+    const remaining = HOT_SPAN - (stamp - lastCalled);
+
+    lastCalled = stamp;
+    if (remaining > 0) {
+      if (++count >= HOT_COUNT) {
+        return args[0];
+      }
+    } else {
+      count = 0;
+    }
+    return func(...args);
+  };
+}
+
+export default shortOut;