[third_party] add polymer 1.2.3

polymer_1.2.3/bower_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);
+  MockInteractions.tap(button);
 });
 ```