Lint for CamelCase type names (Style Guide).

From the [style guide](https://www.dartlang.org/articles/style-guide/):

**DO** name types using UpperCamelCase.

Classes and typedefs should capitalize the first letter of each word (including the first word), and use no separators.

**GOOD:**

```
class SliderMenu {
  // ...
}

class HttpRequest {
  // ...
}

typedef num Adder(num x, num y);
```
