correct global detection

src/Promise.js

@@ -2,8 +2,9 @@
 
 
 var spec = require('./spec')
+var global = require('./global')
 var TypeIsObject = spec.TypeIsObject,
-	WellKnownSymbol = spec.defineSymbol(this.Symbol).WellKnownSymbol,
+	WellKnownSymbol = spec.defineSymbol(global.Symbol).WellKnownSymbol,
 	IsCallable = spec.IsCallable,
 	IsConstructor = spec.IsConstructor,
 	Call = spec.Call,

src/global.js

@@ -0,0 +1,10 @@
+'use strict'
+
+module.exports = function () {
+
+	if (typeof System === 'object' && System && typeof System.global === 'object' && System.global) return System.global
+	if (typeof global === 'object' && global && global.global === global) return global
+	if (typeof window === 'object' && window && window.window === window) return window
+	return new Function('return this')()
+
+}()