Regression: TODOs treated as errors by dartanalyzer as of 1.16.0-dev.1.0

Given:

``` dart
// TODO(ochafik): hello.
main() {}
```

Then `dartanalyzer --strong ~/analyzer.dart` yields:

```
Analyzing [/Users/ochafik/analyzer.dart]...
[error] TODO(ochafik): hello. (/Users/ochafik/analyzer.dart, line 1, col 4)
1 error found.
```

```
dartanalyzer --version
dartanalyzer version 1.16.0-dev.1.0
```

Works fine in 1.16.0-dev.0.0 (emitted as `hint` instead of `error`)
