fix(web components): Fix tests in IE10

@jbdeboer could you please review this PR, I'd like to get this in ASAP as it would allow enabling the tests on IE10.

The [tests pass on IE10](https://travis-ci.org/vicb/angular.dart/builds/33049150) with this update (there is actually a failure in that branch but no more related to wc)

Thanks to @vsavkin for helping me find the root cause of the issue.