Move internal modules to “internal” folder.

.internal/toKey.js

@@ -0,0 +1,21 @@
+import isSymbol from './isSymbol.js';
+
+/** Used as references for various `Number` constants. */
+const INFINITY = 1 / 0;
+
+/**
+ * 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(value) {
+  if (typeof value == 'string' || isSymbol(value)) {
+    return value;
+  }
+  const result = `${ value }`;
+  return (result == '0' && (1 / value) == -INFINITY) ? '-0' : result;
+}
+
+export default toKey;