Trouble writing tasks with TaskArgs params.

Follow-up from: https://github.com/dart-lang/linter/pull/889.

The rub is that using the documented API, with a task written like this:

```dart
@Task('Generate a lint rule stub.')
rule(TaskArgs args) {
String name = args.getOption('name');
generateRule(name, outDir: Directory.current.path);
}
```

I get an exception at runtime:

```
$ grind rule --name=foo

NoSuchMethodError: Class 'GrinderContext' has no instance method 'getOption'.
Receiver: Instance of 'GrinderContext'
Tried calling: getOption("name")
```

Defining the rule to take a context works though:

```dart
@Task('Generate a lint rule stub.')
rule(GrinderContext context) {
TaskArgs args = context.invocation.arguments;
String name = args.getOption('name');
generateRule(name, outDir: Directory.current.path);
}
```

but I don't think that indirection is intended (and there's a `context` getter besides).