Bump to v4.10.0.

_toKey.js

@@ -0,0 +1,14 @@
+var isSymbol = require('./isSymbol');
+
+/**
+ * Converts `value` to a string key if it's not a string or symbol.
+ *
+ * @private
+ * @param {*} value The value to inspect.
+ * @returns {string|symbol} Returns the key.
+ */
+function toKey(key) {
+  return (typeof key == 'string' || isSymbol(key)) ? key : (key + '');
+}
+
+module.exports = toKey;