Remove third_party/polymer from .gitignore

third_party/polymer/v1_0/components/iron-test-helpers/README.md

-# iron-test-helpers
-
-Utility classes to make testing easier.
-
-## Mock Interactions
-
-This is a set of methods to simulate mouse or keyboard interaction with an element. Include `mock-interactions.js` and then use them like so:
-
-```javascript
-test('can be triggered with space', function(done) {
-  button.addEventListener('keydown', function() {
-    done();
-  });
-  MockInteractions.pressSpace(button);
-});
-
-test('can be clicked', function(done) {
-  button.addEventListener('click', function() {
-    done();
-  });
-  MockInteractions.down(button);
-});
-```