Add lint unnecessary_constructor_name - Use A() over A.new()

[natebosch]

https://dart-lang.github.io/linter/lints/unnecessary_constructor_name.html

The constructor tearoffs feature added one bit of useless syntax for consistency sake. We should recommend the lint which avoids the useless .new, and allows it only when it's necessary for a tearoff.

[lrhn]

For now, that's the only necessary use.
Hopefully we'll get generic constructors eventually, then you might also need it for that.

[natebosch]

Hopefully we'll get generic constructors eventually, then you might also need it for that.

I think we'd update the lint to allow the syntax if it is being used to pass type arguments.

[devoncarew]

Approved for recommended.

[devoncarew]

It looks like this was already added in dart-archive/lints#72.