Bump to v4.14.0.

_isFlattenable.js

@@ -1,6 +1,10 @@
+import Symbol from './_Symbol.js';
 import isArguments from './isArguments.js';
 import isArray from './isArray.js';
 
+/** Built-in value references. */
+var spreadableSymbol = Symbol ? Symbol.isConcatSpreadable : undefined;
+
 /**
  * Checks if `value` is a flattenable `arguments` object or array.
  *
@@ -9,7 +13,8 @@ import isArray from './isArray.js';
  * @returns {boolean} Returns `true` if `value` is flattenable, else `false`.
  */
 function isFlattenable(value) {
-  return isArray(value) || isArguments(value);
+  return isArray(value) || isArguments(value) ||
+    !!(spreadableSymbol && value && value[spreadableSymbol])
 }
 
 export default isFlattenable;