Void is not required to be `null` anymore.

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index e00640e3ab021ababcfcce05b187fa01b2f23252..a6ba79ffd30daf4695c7a0ed0c9d35acc87397cd 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -3820,7 +3820,6 @@ bool Class::TypeTestNonRecursive(const Class& cls,
   Zone* zone = Thread::Current()->zone();
   Class& thsi = Class::Handle(zone, cls.raw());
   while (true) {
-    ASSERT(!thsi.IsVoidClass());
     // Check for DynamicType.
     // Each occurrence of DynamicType in type T is interpreted as the dynamic
     // type, a supertype of all types.
@@ -3839,10 +3838,15 @@ bool Class::TypeTestNonRecursive(const Class& cls,
       return test_kind == Class::kIsSubtypeOf;
     }
     // Check for ObjectType. Any type that is not NullType or DynamicType
-    // (already checked above), is more specific than ObjectType.
-    if (other.IsObjectClass()) {
+    // (already checked above), is more specific than ObjectType/VoidType.
+    if (other.IsObjectClass() || other.IsVoidClass()) {
       return true;
     }
+    // If other is neither Object, dynamic or void, then ObjectType/VoidType
+    // can't be a subtype of other.
+    if (thsi.IsObjectClass() || thsi.IsVoidClass()) {
+      return false;
+    }
     // Check for reflexivity.
     if (thsi.raw() == other.raw()) {
       const intptr_t num_type_params = thsi.NumTypeParameters();
@@ -15830,7 +15834,7 @@ bool Instance::IsInstanceOf(
   ASSERT(!other.IsMalformed());
   ASSERT(!other.IsMalbounded());
   if (other.IsVoidType()) {
-    return false;
+    return true;
   }
   Zone* zone = Thread::Current()->zone();
   const Class& cls = Class::Handle(zone, clazz());
@@ -16648,9 +16652,12 @@ bool AbstractType::TypeTest(TypeTestKind test_kind,
     return false;
   }
   // Any type is a subtype of (and is more specific than) Object and dynamic.
+  // As of Dart 1.24, void is dynamically treated like Object (except when
+  // comparing function-types).
   // As of Dart 1.5, the Null type is a subtype of (and is more specific than)
   // any type.
-  if (other.IsObjectType() || other.IsDynamicType() || IsNullType()) {
+  if (other.IsObjectType() || other.IsDynamicType() || other.IsVoidType() ||
+      IsNullType()) {
     return true;
   }
   Zone* zone = Thread::Current()->zone();