kangax/es5-compat-table — comment on commit 75c707f4b36747a917bdc2396be8e695f0fe5b32
File: es6/index.html line 515
By DavidBruant:

I don't know if it's in any spec, but I think the "Proxy.for" syntax is being abandonned in favor of the more natural:

var p = new Proxy(target, handler).
So a more relevant test would be: (typeof Proxy === 'function')