Move Null class out of the VM isolate.

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index a1455845b9906e9be6c35cd6c51c22d6d6e3b29a..42db0cbb384228c7a05a383c99c76af835361017 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -405,7 +405,6 @@ class Object {
   }
 
   static RawClass* class_class() { return class_class_; }
-  static RawClass* null_class() { return null_class_; }
   static RawClass* dynamic_class() { return dynamic_class_; }
   static RawClass* void_class() { return void_class_; }
   static RawClass* unresolved_class_class() { return unresolved_class_class_; }
@@ -553,7 +552,6 @@ class Object {
   static RawObject* null_;
 
   static RawClass* class_class_;  // Class of the Class vm object.
-  static RawClass* null_class_;  // Class of the null object.
   static RawClass* dynamic_class_;  // Class of the 'dynamic' type.
   static RawClass* void_class_;  // Class of the 'void' type.
   static RawClass* unresolved_class_class_;  // Class of UnresolvedClass.
@@ -3745,9 +3743,7 @@ class AbstractType : public Instance {
   }
 
   // Check if this type represents the 'Null' type.
-  bool IsNullType() const {
-    return HasResolvedTypeClass() && (type_class() == Object::null_class());
-  }
+  bool IsNullType() const;
 
   // Check if this type represents the 'void' type.
   bool IsVoidType() const {