Failing dart forms with multiple attribute selectors

Here's a branch of the code that is failing.

## How to reproduce the error
1. Ensure both NgDirective selectors are present for `required`.

```
@NgDirective(selector: '[ng-model][required]')
@NgDirective(selector: '[ng-model][ng-required]', map: const {'ng-required': '=>required'})
```

2. Run the tests and you'll see that it doesn't work.

## What I found when debugging

1. If you remove this from `ng_src_boolean.dart`

```
@NgDirective(selector: '[ng-required]', map: const {'ng-required': '=>required'})
```

Then both work fine.

Looking into the problem further I was able to debug it down to the `class _SelectorPart` which is causing the bug. I don't understand the code fully just yet to see what is going on. Please have a look.