Adjust test to allow type variable as expression

This CL adjusts the test
language/constants_2018/constant_types_test
such that it expects no error when a type variable is used as a
potentially constant expression.

This feature was added for consistency as part of the constructor-
tearoffs feature bundle (because it can now be expressed anyway).

Change-Id: I5b58f2439699b24e3e3036cb0e3a0fcc9430d4c0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/214644
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Erik Ernst <[email protected]>

Author: eernstg

tests/language/constants_2018/constant_types_test.dart

@@ -4,7 +4,7 @@
 
 // Tests that in some positions only constant types are allowed, so not
 // type parameters. But in other positions potentially constant types are
-// allowed, so type parameters.
+// allowed, including type parameters.
 
 import "package:expect/expect.dart";
 
@@ -24,7 +24,7 @@ class T<X> {
   ;
   const T.test4()
       : value = null //
-            ?? X //# 04: compile-time error
+            ?? X //# 04: ok
   ;
 }