[vm/nnbd/bytecode] Distinguish function types with different nullabilities in bytecode generator

Fixes #42807

Change-Id: I054b79c5b199fbc53e512caf8e41ddbcb79d3675
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/155682
Reviewed-by: Régis Crelier <[email protected]>
Commit-Queue: Alexander Markov <[email protected]>

Author: alexmarkov

pkg/vm/lib/bytecode/object_table.dart

@@ -1149,6 +1149,7 @@ class _FunctionTypeHandle extends _TypeHandle {
     hash = _combineHashes(hash, listHashCode(positionalParams));
     hash = _combineHashes(hash, listHashCode(namedParams));
     hash = _combineHashes(hash, returnType.hashCode);
+    hash = _combineHashes(hash, nullability.index);
     return hash;
   }
 
@@ -1159,7 +1160,8 @@ class _FunctionTypeHandle extends _TypeHandle {
       this.numRequiredParams == other.numRequiredParams &&
       listEquals(this.positionalParams, other.positionalParams) &&
       listEquals(this.namedParams, other.namedParams) &&
-      this.returnType == other.returnType;
+      this.returnType == other.returnType &&
+      this.nullability == other.nullability;
 
   @override
   String toString() {