Attempt to fix the linker issues by wrapping the template call with a helper

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index 18526813d0046934248b154471c878489e14a9c1..6af368bf017b38dc388b910a256aa04da9ae3dfb 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -1511,6 +1511,9 @@ class Class : public Object {
   // Allocate an instance class which has a VM implementation.
   template <class FakeInstance> static RawClass* New(intptr_t id);
 
+  // Helper that calls 'Class::New<Instance>(kIllegalCid)'.
+  static RawClass* NewInstanceClass();
+
   // Check the subtype or 'more specific' relationship.
   bool TypeTest(TypeTestKind test_kind,
                 const TypeArguments& type_arguments,