Add context menu option to "comment out widget"

When I'm hacking or prototyping with Flutter I occasionally want to comment out one or a few Widgets within a Widget tree. This allows me to verify that certain effects are happening the way that I think they are by temporarily commenting out unrelated Widgets.

Example:

Start with:
```dart
new Widget1(
child: new Widget2(
child: new Widget3(
// params
),
),
);
```

Comment out the 2nd widget:
```dart
new Widget1(
//child: new Widget2(
child: new Widget3(
// params
),
//),
);
```

I'd like a context menu option that allows me to comment out one or more Widgets in the middle of a tree. This can't be accomplished with the traditional "CMD+/" because the closing braces won't be commented out and then it becomes very difficult to find the corresponding braces.