Bump to v4.10.0.

_toKey.js

@@ -0,0 +1,15 @@
+define(['./isSymbol'], function(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 + '');
+  }
+
+  return toKey;
+});