Polymer 1 5

Updating the polymer dart examples to 0.15.1 and incorporating Siggi's comments. Also updating the comment style to use "///" instead of "/*" for doc comments.

@kwalrath @sigmundch 
I'd change this and remove the DOCTYPE, html, head, and body tags so it simply looks like:

```html
<link rel="import" href="packages/polymer/polymer.html">
<polymer-element name="tute-slambook-form" extends="form">
   ...
```