[vm/nnbd] Fix element type nullability check in _GrowableList.length=

When growing an array, we're checking if element type is nullable.
The problem is that 'null is! T' is true for legacy types T
(other than top types and Null), so it throws for List<int*>.

In order to be able to grow arrays with all element types which can
hold null (including legacy element types) this test is changed to

      null as T;

which throws type error if T cannot hold null.

Change-Id: Icf0a397109945b049b4aaead4c7eab11d903c45b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/134790
Reviewed-by: Liam Appelbe <[email protected]>
Reviewed-by: Régis Crelier <[email protected]>
Commit-Queue: Alexander Markov <[email protected]>

Author: alexmarkov

sdk_nnbd/lib/_internal/vm/lib/growable_array.dart

@@ -116,10 +116,8 @@ class _GrowableList<T> extends ListBase<T> {
     int old_capacity = _capacity;
     int new_capacity = new_length;
     if (new_capacity > old_capacity) {
-      if (null is! T) {
-        throw UnsupportedError(
-            "Cannot grow array with non-nullable element type");
-      }
+      // Verify that element type is nullable.
+      null as T;
       _grow(new_capacity);
       _setLength(new_length);
       return;