Fix test with syntax error

This CL changes `late x;` to `late var x;` in a test. Apparently, this
syntax was considered correct and the front end and analyzer accept it.
However, it is a syntax error, and this CL corrects it. The fact that
the tools accept it is reported as SDK issue #43811.

Change-Id: Iaa2fe56fe99d3702345368a54a7389a59288515c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/167804
Reviewed-by: Nate Bosch <[email protected]>
Commit-Queue: Erik Ernst <[email protected]>

Author: eernstg

tests/language/nnbd/inference/late_variable_type_for_promotion_test.dart

@@ -95,7 +95,7 @@ void nullableUninitializedWithType() {
 }
 
 void uninitializedUntyped() {
-  late x;
+  late var x;
   x = 0;
   x.expectStaticType<Exactly<dynamic>>();
   // Attempting to promote to `int?` should be ok, since `int?` is a subtype of