Fix event analytics

Staged at:
https://events-dot-dart-lang.appspot.com/

It seems like a straightforward conversion, but I'd appreciate
a careful check. Here's the page with the upgrade instructions:

https://developers.google.com/analytics/devguides/collection/upgrade/reference/gajs-analyticsjs#virtual-pageviews

R=sethladd@google.com

Review URL: https://chromiumcodereview.appspot.com/327693004