fix(SymbolShim): Symbol polyfill is a function

Symbol is supposed to be a function that takes a description and returns a unique symbol
such that `Symbol('foo') !== Symbol('foo')`. The polyfill therefor returns a string that has
a unique identifier concatenated to it: `@@symbol(description):1` `@@symbol(description):2` etc

fixes #988

spec/util/SymbolShim-spec.js

@@ -5,7 +5,19 @@ var Rx = require('../../dist/cjs/Rx');
 var polyfillSymbol = SymbolShim.polyfillSymbol;
 var ensureIterator = SymbolShim.ensureIterator;
 
-describe('SymbolShim', function () {
+describe('SymbolShim.polyfillSymbol', function () {
+  it('should polyfill Symbol to be a function that returns a primitive that is unique', function () {
+    var Symbol = polyfillSymbol({ });
+
+    expect(typeof Symbol).toBe('function');
+    var x = Symbol('test');
+    var y = Symbol('test');
+    expect(x !== y).toBe(true); // should be obvious, but this is the important part.
+
+    expect(x).toBe('@@Symbol(test):0');
+    expect(y).toBe('@@Symbol(test):1');
+  });
+
   it('should setup symbol if root does not have it', function () {
     var root = {};
 

src/util/SymbolShim.ts

@@ -14,9 +14,13 @@ export function ensureFor(Symbol) {
   }
 }
 
+let id = 0;
+
 export function ensureSymbol(root) {
   if (!root.Symbol) {
-    root.Symbol = {};
+    root.Symbol = function symbolFuncPolyfill(description) {
+      return `@@Symbol(${description}):${id++}`;
+    };
   }
   return root.Symbol;
 }