[vm] Fix function type testing in NNBD legacy mode.

This fixes co19_2/Language/Generics/typedef_A04_t06 in legacy mode.

Note: There's still an underlying issue that we'll probably run into
when switching to strong mode:
#40258

Bug: #40171
Change-Id: I335addf887dabdb0ae96aca95f187974dac50215
Fixes: #40171
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/132702
Commit-Queue: Liam Appelbe <[email protected]>
Reviewed-by: Alexander Markov <[email protected]>

Author: liamappelbe

runtime/vm/object.cc

@@ -7439,7 +7439,10 @@ bool Function::HasSameTypeParametersAndBounds(const Function& other) const {
       ASSERT(bound.IsFinalized());
       other_bound = other_type_param.bound();
       ASSERT(other_bound.IsFinalized());
-      if (!bound.Equals(other_bound)) {
+      // TODO(dartbug.com/40259): Treat top types as equivalent and disregard
+      // nullability in weak mode.
+      const bool syntactically = !FLAG_strong_non_nullable_type_checks;
+      if (!bound.IsEquivalent(other_bound, syntactically)) {
         return false;
       }
     }