Bump to v4.15.0.

_toSource.js

@@ -1,5 +1,8 @@
+/** Used for built-in method references. */
+var funcProto = Function.prototype;
+
 /** Used to resolve the decompiled source of functions. */
-var funcToString = Function.prototype.toString;
+var funcToString = funcProto.toString;
 
 /**
  * Converts `func` to its source code.