In generic type definition, optimize self references.

[vsmenon]

This is an optimization that will happen to avoid one case of #253 that we're hitting.

Here's a simplification:

class Bar<T> {
}

class Foo<T> extends Bar<Foo<T>> {
}

void main() {
  print(new Foo<int>());
}

It hits an infinite loop today. Here's the generate JS:

  generic.Bar$ = dart.generic(T => {
    class Bar extends core.Object {}
    return Bar;
  });
  generic.Bar = generic.Bar$();
  generic.Foo$ = dart.generic(T => {
    class Foo extends generic.Bar {}
    dart.setBaseClass(Foo, generic.Bar$(generic.Foo$(T)));
    return Foo;
  });
  generic.Foo = generic.Foo$();

The recursive call to generic.Foo$(T) is causing the problem. We could safely replace it with Foo, which is already defined in scope here.