Fixing Web UI article: replacing window.setInterval() with Timer.periodic() constructor

Second example of "One-way data binding" section of the Web UI article did not comply with the linked full code [1], and it wasn't working anymore [2]:

```
Class 'Window' has no instance method 'setInterval'.

NoSuchMethodError : method not found: 'setInterval'
Receiver: Instance of 'Window'
Arguments: [Closure: () => dynamic, 1000]
```
(SDK 0.6.17.2_r26023)

References:
[1] https://github.com/dart-lang/web-ui/blob/master/example/explainer/counter.html
[2] [Breaking change: Removal of window.setTimeout and window.setInterval](https://groups.google.com/a/dartlang.org/forum/#!msg/misc/9h7pxLbv5fU/4pn9PJUvoTAJ)