Use {} when Window, Global and Self are undefined (#663)

Author: easysoft2k15

src/fable/Fable.Core/ts/Symbol.ts

@@ -10,7 +10,7 @@ const fableGlobal: {
   const globalObj =
     typeof window !== "undefined" ? window
     : (typeof global !== "undefined" ? global
-    : (typeof self !== "undefined" ? self : null));
+    : (typeof self !== "undefined" ? self : {}));
   if (typeof globalObj.__FABLE_CORE__ === "undefined") {
     globalObj.__FABLE_CORE__ = {
       types: new Map<string, FunctionConstructor>(),