Added ability to parse javascript getters and convert them into dart getters.

Also applied this to all core elements so you can see the changes.
it seems we don't have any setters in the .js code, but I worry that if they add one this will keep things as readOnly regardless. For example, in this code:
```
// @property  ...
get foo() { }
set foo(x) {}
```

do we end up with a read-only property?