add test for core-animated-pages

it's a good question - I'm sure if you ask others you'll get a different answer :-) ... I personally avoid it on the core libraries, except for 'dart:js' (but that might be because the 'context' variable is a bit too generic). Even in that case, I sometimes do:

```dart
import 'dart:js' hide context;
import 'dart:js' as js show context;
```

As for packages - I normally only add a prefix when it's not clear (the names it exposes are too generic), when it creates a conflict, or when it reads better.  For example, the `path` package has very generic top-level methods and it reads better to use it with a prefix: `path.join(a, b)` instead of just (`join(a, b)`).

I tend to use `show` when I'm only using a couple definitions from an import. I also use it when I want to make it more obvious where a symbol comes from, without having to have a prefix on every place I use that symbol. This tends to be especially useful when the imported name is very long, so adding a prefix would be too long.