fix(SymbolShim): ensure for function even if Symbol already exists

In some engines, Symbol exists but does not impelment the for method.

fixes #999

spec/util/SymbolShim-spec.js

@@ -25,6 +25,18 @@ describe('SymbolShim', function () {
  expect(test).toBe('@@test');
  });
 
+ it('should add a for method even if Symbol already exists but does not have for', function () {
+ var root = {
+ Symbol: {}
+ };
+ var result = polyfillSymbol(root);
+
+ expect(typeof result.for).toBe('function');
+
+ var test = result.for('test');
+ expect(test).toBe('@@test');
+ });
+
  describe('when symbols exists on root', function () {
  it('should use symbols from root', function () {
  var root = {
@@ -59,17 +71,15 @@ describe('SymbolShim', function () {
  expect(result.observable).toBe('@@observable');
  });
  });
-});
 
-describe('ensureIterator', function () {
- it('should patch root using for symbol if exist', function () {
+ it('should patch root using Symbol.for if exist', function () {
  var root = {
  Symbol: {
  for: function (x) { return x; }
  }
  };
- ensureIterator(root.Symbol, root);
- expect(root.Symbol.iterator).toBe(root.Symbol.for('iterator'));
+ var result = polyfillSymbol(root);
+ expect(result.iterator).toBe(root.Symbol.for('iterator'));
  });
 
  it('should patch using Set for mozilla bug', function () {
@@ -82,8 +92,8 @@ describe('ensureIterator', function () {
  Symbol: {}
  };
 
- ensureIterator(root.Symbol, root);
- expect(root.Symbol.iterator).toBe('@@iterator');
+ var result = polyfillSymbol(root);
+ expect(result.iterator).toBe('@@iterator');
  });
 
  it('should patch using map for es6-shim', function () {
@@ -95,7 +105,7 @@ describe('ensureIterator', function () {
  root.Map.prototype.key = 'iteratorValue';
  root.Map.prototype.entries = 'iteratorValue';
 
- ensureIterator(root.Symbol, root);
- expect(root.Symbol.iterator).toBe('key');
+ var result = polyfillSymbol(root);
+ expect(result.iterator).toBe('key');
  });
 });

src/util/SymbolShim.ts

@@ -4,14 +4,19 @@ export function polyfillSymbol(root) {
  const Symbol = ensureSymbol(root);
  ensureIterator(Symbol, root);
  ensureObservable(Symbol);
+ ensureFor(Symbol);
  return Symbol;
 }
 
+export function ensureFor(Symbol) {
+ if (!Symbol.for) {
+ Symbol.for = symbolForPolyfill;
+ }
+}
+
 export function ensureSymbol(root) {
  if (!root.Symbol) {
- root.Symbol = {
- for: symbolForPolyfill
- };
+ root.Symbol = {};
  }
  return root.Symbol;
 }