Expand "is" for Types

In the following code:

```dart
void main() async {
MyClass c = MyClass();

print(c is MyInterface); // prints true
print(MyClass is MyInterface); // prints false
print(MyClass is MyClass); // prints false
}

class MyClass implements MyInterface {
String s;
}

class MyInterface {
String s;
}
```

`MyClass` implements `MyInterface` but I can't test this with `is` directly using types, only with a variable.

Since Dart generics are very limited on multiple inheritance bounds, I was trying to make my code more secure by adding checks like following before doing some operations:

```dart
assert(V is MyInterface);
```

But since I don't have an actual variable of type `V` to test (I am searching a value using the type V as the key), this code throws an exception.

Start with the issue's Dart examples and compare the behavior of `is` on an instance with the proposed checks on `MyClass` and type parameter `V`. Determine the intended semantics and constraints for testing type relationships without a value; done would require an agreed language-design direction or specification change.

Written by the indexing model from the issue text.