Allow type variables, not just type literals, as type parameters

[justinfagnani]

For type checking on certain factory functions, it'd be very nice to allow specifying type parameters from variables, not just literals.

Example:

class A<T> {
  String toString() => "A<$T>";
}

A makeA(Type t) => new A<t>();

main() {
  var a = makeA(String);
  print("a = $a");
}

[lrhn]

That would be a different way to get type-parametereized methods.
An alternative syntax could be:
...
  A makeA<S>() => new A<S>();
  main() {
   var a = makeA<String>();
   print("a = $a");
  }
...

This has the advantage that the compiler knows that S is a type, where your suggested syntax can't rely on the "Type" type-annotation in unchecked mode, and should have to dynamically check that "t" is actually a type. It could be any variable (and then, why not any expression, except that it would make parsing very very hard).

So, is this a duplicate of issue #254, or is the different syntax important?

[gbracha]

I see this as a duplicate of issue #254. The specific suggestion has pros and cons. In an ideal world, I would implement generics using first class type objects passed as ordinary parameters and dispense with the entire edifice of special syntax etc. However, for Dart I think this is rather unlikely. So the more probable course of action would involve traditional generic methods as suggested in issue #254. Even that is debatable.

---

Added Duplicate label.
Marked as being merged into #254.

[justinfagnani]

I know that this overlaps with issue #254 somewhat, but sometimes you do have a Type in a variable, and even with generic methods you'd need a way to place the variable in a type parameter position. As long as we have semi-first-class type values this will come up in some cases.

In one of my cases I'm using Types as keys in a map. If I want to construct a new type parameterized object from a key, generic methods don't help.

I would propose un-duping this issue.