Adding higher-order type constructor to dart

While working on a functional programming package for dart I found that dart does not support **higher-order type constructors** (**higher-kinded type**).

For example, a typical [`Foldable`](https://github.com/typelevel/cats/blob/ddc8a0a2039bd07206b86754f22accb605f13575/core/src/main/scala/cats/Foldable.scala#L34) typeclass expects a _generic parameter_ `F` that itself can have **another** generic parameter.

In scala, [it is already possible](https://stackoverflow.com/a/6417328/7033357) to do that with the following syntax:
```scala
trait Foldable[F[_]]
```
As a concrete example, the type `F` can be a `List` and `[_]` means that the `List` can itself accept any type (e.g. `List`). This feature allows defining functions like the following:
```scala
def foldLeft[A, B](fa: F[A], b: B)(f: (B, A) => B): B
```
If `F` is `List` then:
```dart
B foldLeft(List fa, B b, B Function(B, A) f);
```

This is not possible currently in dart. The `fa` parameter cannot itself accept another generic parameter.
```dart
/// Not valid dart code!
abstract class Foldable> {
B foldLeft(F fa, B b, B Function(B, A) f);
}

/// Not valid dart code!
abstract class Foldable {
// Error: The type 'F' is declared with 0 type parameters, but 1 type arguments were given.
B foldLeft(F fa, B b, B Function(B, A) f);
}
```

Is it possible to implement this feature in the dart language?

Start with the Foldable examples and the proposed Dart syntax in the issue, comparing them with the linked Scala example. Done means establishing whether higher-kinded types can be added to Dart and defining the required generic behavior and syntax.

Written by the indexing model from the issue text.