Allow type parameters on constructors

[Hixie]

I have a class of type Foo<T> which I want to construct using a source of R values and a callback that can convert Rs to Ts.

   typedef T Converter<R, T>(R value);
   class Foo<T> {
     factory Foo.convert<R>(Bar<R> source, Converter<R, T> converter) {
       // ...
     }
   }

This fails with Error: Constructors can't have type parameters.

[Hixie]

(Using a static method instead fails with #34415.)

[eernstg]

This has been in the pipeline for a while, it just didn't make it into Dart 2. We have https://github.com/dart-lang/sdk/issues/26391 which is used to discuss (and push for) that feature, so I'll close this one as a duplicate thereof.