Check whether type parameters are loaded before emitting

test/codegen/functions.dart

+List<Foo> bootstrap() {
+  return <Foo>[new Foo()];
+}
+
+typedef B A2B<A, B>(A x);
+
+A2B<Foo, Foo> id(A2B<Foo, Foo> f) => f;
+
+class Foo {
+}
+
+void main() {
+  print(bootstrap()[0]);
+}